Fix configure's AC_CHECK_DECLS tests to work correctly with clang.

The test case that Autoconf uses to discover whether a function has
been declared doesn't work reliably with clang, because clang reports
a warning not an error if the name is a known built-in function.
On some platforms, this results in a lot of compile-time warnings about
strlcpy and related functions not having been declared.

There is a fix for this (by Noah Misch) in the upstream Autoconf sources,
but since they've not made a release in years and show no indication of
doing so anytime soon, let's just absorb their fix directly.  We can
revert this when and if we update to a newer Autoconf release.

Back-patch to all supported branches.

Discussion: https://postgr.es/m/26819.1542515567@sss.pgh.pa.us

aclocal.m4

@@ -4,6 +4,7 @@ m4_include([config/ax_prog_perl_modules.m4])
 m4_include([config/ax_pthread.m4])
 m4_include([config/c-compiler.m4])
 m4_include([config/c-library.m4])
+m4_include([config/check_decls.m4])
 m4_include([config/docbook.m4])
 m4_include([config/general.m4])
 m4_include([config/libtool.m4])

config/check_decls.m4

+# config/check_decls.m4
+
+# This file redefines the standard Autoconf macro _AC_CHECK_DECL_BODY,
+# and adds a supporting function _AC_UNDECLARED_WARNING, to make
+# AC_CHECK_DECLS behave correctly when checking for built-in library
+# functions with clang.
+
+# This is based on commit 82ef7805faffa151e724aa76c245ec590d174580
+# in the Autoconf git repository.  We can drop it if they ever get
+# around to releasing a new version of Autoconf.  In the meantime,
+# it's distributed under Autoconf's license:
+
+# This file is part of Autoconf.  This program is free
+# software; you can redistribute it and/or modify it under the
+# terms of the GNU General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# Under Section 7 of GPL version 3, you are granted additional
+# permissions described in the Autoconf Configure Script Exception,
+# version 3.0, as published by the Free Software Foundation.
+#
+# You should have received a copy of the GNU General Public License
+# and a copy of the Autoconf Configure Script Exception along with
+# this program; see the files COPYINGv3 and COPYING.EXCEPTION
+# respectively.  If not, see <http://www.gnu.org/licenses/>.
+
+# Written by David MacKenzie, with help from
+# Franc,ois Pinard, Karl Berry, Richard Pixley, Ian Lance Taylor,
+# Roland McGrath, Noah Friedman, david d zuhn, and many others.
+
+
+# _AC_UNDECLARED_WARNING
+# ----------------------
+# Set ac_[]_AC_LANG_ABBREV[]_decl_warn_flag=yes if the compiler uses a warning,
+# not a more-customary error, to report some undeclared identifiers.  Fail when
+# an affected compiler warns also on valid input.  _AC_PROG_PREPROC_WORKS_IFELSE
+# solves a related problem.
+AC_DEFUN([_AC_UNDECLARED_WARNING],
+[# The Clang compiler raises a warning for an undeclared identifier that matches
+# a compiler builtin function.  All extant Clang versions are affected, as of
+# Clang 3.6.0.  Test a builtin known to every version.  This problem affects the
+# C and Objective C languages, but Clang does report an error under C++ and
+# Objective C++.
+#
+# Passing -fno-builtin to the compiler would suppress this problem.  That
+# strategy would have the advantage of being insensitive to stray warnings, but
+# it would make tests less realistic.
+AC_CACHE_CHECK([how $[]_AC_CC[] reports undeclared, standard C functions],
+[ac_cv_[]_AC_LANG_ABBREV[]_decl_report],
+[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [(void) strchr;])],
+  [AS_IF([test -s conftest.err], [dnl
+    # For AC_CHECK_DECL to react to warnings, the compiler must be silent on
+    # valid AC_CHECK_DECL input.  No library function is consistently available
+    # on freestanding implementations, so test against a dummy declaration.
+    # Include always-available headers on the off chance that they somehow
+    # elicit warnings.
+    AC_COMPILE_IFELSE([AC_LANG_PROGRAM([dnl
+#include <float.h>
+#include <limits.h>
+#include <stdarg.h>
+#include <stddef.h>
+extern void ac_decl (int, char *);],
+[@%:@ifdef __cplusplus
+  (void) ac_decl ((int) 0, (char *) 0);
+  (void) ac_decl;
+@%:@else
+  (void) ac_decl;
+@%:@endif
+])],
+      [AS_IF([test -s conftest.err],
+	[AC_MSG_FAILURE([cannot detect from compiler exit status or warnings])],
+	[ac_cv_[]_AC_LANG_ABBREV[]_decl_report=warning])],
+      [AC_MSG_FAILURE([cannot compile a simple declaration test])])],
+    [AC_MSG_FAILURE([compiler does not report undeclared identifiers])])],
+  [ac_cv_[]_AC_LANG_ABBREV[]_decl_report=error])])
+
+case $ac_cv_[]_AC_LANG_ABBREV[]_decl_report in
+  warning) ac_[]_AC_LANG_ABBREV[]_decl_warn_flag=yes ;;
+  *) ac_[]_AC_LANG_ABBREV[]_decl_warn_flag= ;;
+esac
+])# _AC_UNDECLARED_WARNING
+
+# _AC_CHECK_DECL_BODY
+# -------------------
+# Shell function body for AC_CHECK_DECL.
+m4_define([_AC_CHECK_DECL_BODY],
+[  AS_LINENO_PUSH([$[]1])
+  # Initialize each $ac_[]_AC_LANG_ABBREV[]_decl_warn_flag once.
+  AC_DEFUN([_AC_UNDECLARED_WARNING_]_AC_LANG_ABBREV,
+	   [_AC_UNDECLARED_WARNING])dnl
+  AC_REQUIRE([_AC_UNDECLARED_WARNING_]_AC_LANG_ABBREV)dnl
+  [as_decl_name=`echo $][2|sed 's/ *(.*//'`]
+  [as_decl_use=`echo $][2|sed -e 's/(/((/' -e 's/)/) 0&/' -e 's/,/) 0& (/g'`]
+  AC_CACHE_CHECK([whether $as_decl_name is declared], [$[]3],
+  [ac_save_werror_flag=$ac_[]_AC_LANG_ABBREV[]_werror_flag
+  ac_[]_AC_LANG_ABBREV[]_werror_flag="$ac_[]_AC_LANG_ABBREV[]_decl_warn_flag$ac_[]_AC_LANG_ABBREV[]_werror_flag"
+  AC_COMPILE_IFELSE([AC_LANG_PROGRAM([$[]4],
+[@%:@ifndef $[]as_decl_name
+@%:@ifdef __cplusplus
+  (void) $[]as_decl_use;
+@%:@else
+  (void) $[]as_decl_name;
+@%:@endif
+@%:@endif
+])],
+		   [AS_VAR_SET([$[]3], [yes])],
+		   [AS_VAR_SET([$[]3], [no])])
+  ac_[]_AC_LANG_ABBREV[]_werror_flag=$ac_save_werror_flag])
+  AS_LINENO_POP
+])# _AC_CHECK_DECL_BODY

configure

@@ -1756,13 +1756,16 @@ fi
 ac_fn_c_check_decl ()
 {
   as_lineno=${as_lineno-"$1"} as_lineno_stack=as_lineno_stack=$as_lineno_stack
-  as_decl_name=`echo $2|sed 's/ *(.*//'`
+  # Initialize each $ac_[]_AC_LANG_ABBREV[]_decl_warn_flag once.
+      as_decl_name=`echo $2|sed 's/ *(.*//'`
   as_decl_use=`echo $2|sed -e 's/(/((/' -e 's/)/) 0&/' -e 's/,/) 0& (/g'`
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $as_decl_name is declared" >&5
 $as_echo_n "checking whether $as_decl_name is declared... " >&6; }
 if eval \${$3+:} false; then :
   $as_echo_n "(cached) " >&6
 else
+  ac_save_werror_flag=$ac_c_werror_flag
+  ac_c_werror_flag="$ac_c_decl_warn_flag$ac_c_werror_flag"
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 $4
@@ -1787,6 +1790,7 @@ else
   eval "$3=no"
 fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  ac_c_werror_flag=$ac_save_werror_flag
 fi
 eval ac_res=\$$3
 	       { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_res" >&5
@@ -4990,11 +4994,100 @@ fi
   test -n "$AWK" && break
 done
 
+# The Clang compiler raises a warning for an undeclared identifier that matches
+# a compiler builtin function.  All extant Clang versions are affected, as of
+# Clang 3.6.0.  Test a builtin known to every version.  This problem affects the
+# C and Objective C languages, but Clang does report an error under C++ and
+# Objective C++.
+#
+# Passing -fno-builtin to the compiler would suppress this problem.  That
+# strategy would have the advantage of being insensitive to stray warnings, but
+# it would make tests less realistic.
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking how $CC reports undeclared, standard C functions" >&5
+$as_echo_n "checking how $CC reports undeclared, standard C functions... " >&6; }
+if ${ac_cv_c_decl_report+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+(void) strchr;
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  if test -s conftest.err; then :
+      # For AC_CHECK_DECL to react to warnings, the compiler must be silent on
+    # valid AC_CHECK_DECL input.  No library function is consistently available
+    # on freestanding implementations, so test against a dummy declaration.
+    # Include always-available headers on the off chance that they somehow
+    # elicit warnings.
+    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <float.h>
+#include <limits.h>
+#include <stdarg.h>
+#include <stddef.h>
+extern void ac_decl (int, char *);
+int
+main ()
+{
+#ifdef __cplusplus
+  (void) ac_decl ((int) 0, (char *) 0);
+  (void) ac_decl;
+#else
+  (void) ac_decl;
+#endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  if test -s conftest.err; then :
+  { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "cannot detect from compiler exit status or warnings
+See \`config.log' for more details" "$LINENO" 5; }
+else
+  ac_cv_c_decl_report=warning
+fi
+else
+  { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "cannot compile a simple declaration test
+See \`config.log' for more details" "$LINENO" 5; }
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+else
+  { { $as_echo "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+$as_echo "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "compiler does not report undeclared identifiers
+See \`config.log' for more details" "$LINENO" 5; }
+fi
+else
+  ac_cv_c_decl_report=error
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_c_decl_report" >&5
+$as_echo "$ac_cv_c_decl_report" >&6; }
+
+case $ac_cv_c_decl_report in
+  warning) ac_c_decl_warn_flag=yes ;;
+  *) ac_c_decl_warn_flag= ;;
+esac
+
 if test "$with_llvm" = yes; then :
 
 
 
 
+
   if test -z "$LLVM_CONFIG"; then
   for ac_prog in llvm-config llvm-config-7 llvm-config-6.0 llvm-config-5.0 llvm-config-4.0 llvm-config-3.9
 do
@@ -5250,7 +5343,8 @@ _ACEOF
 
 
 
-fi
+
+fi # fi
 
 
 unset CFLAGS
@@ -15197,7 +15291,8 @@ esac
 # posix_fadvise() is a no-op on Solaris, so don't incur function overhead
 # by calling it, 2009-04-02
 # http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/lib/libc/port/gen/posix_fadvise.c
-if test "$PORTNAME" != "solaris"; then
+if test "$PORTNAME" != "solaris"; then :
+
 for ac_func in posix_fadvise
 do :
   ac_fn_c_check_func "$LINENO" "posix_fadvise" "ac_cv_func_posix_fadvise"
@@ -15221,7 +15316,8 @@ cat >>confdefs.h <<_ACEOF
 #define HAVE_DECL_POSIX_FADVISE $ac_have_decl
 _ACEOF
 
-fi
+
+fi # fi
 
 ac_fn_c_check_decl "$LINENO" "fdatasync" "ac_cv_have_decl_fdatasync" "#include <unistd.h>
 "

configure.in

@@ -392,7 +392,9 @@ PGAC_ARG_BOOL(with, llvm, no, [build with LLVM based JIT support],
               [AC_DEFINE([USE_LLVM], 1, [Define to 1 to build with LLVM based JIT support. (--with-llvm)])])
 AC_SUBST(with_llvm)
 dnl must use AS_IF here, else AC_REQUIRES inside PGAC_LLVM_SUPPORT malfunctions
-AS_IF([test "$with_llvm" = yes], [PGAC_LLVM_SUPPORT()])
+AS_IF([test "$with_llvm" = yes], [
+  PGAC_LLVM_SUPPORT()
+]) # fi
 
 
 unset CFLAGS
@@ -1640,10 +1642,11 @@ esac
 # posix_fadvise() is a no-op on Solaris, so don't incur function overhead
 # by calling it, 2009-04-02
 # http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/lib/libc/port/gen/posix_fadvise.c
-if test "$PORTNAME" != "solaris"; then
+dnl must use AS_IF here, else AC_REQUIRES inside AC_CHECK_DECLS malfunctions
+AS_IF([test "$PORTNAME" != "solaris"], [
 AC_CHECK_FUNCS(posix_fadvise)
 AC_CHECK_DECLS(posix_fadvise, [], [], [#include <fcntl.h>])
-fi
+]) # fi
 
 AC_CHECK_DECLS(fdatasync, [], [], [#include <unistd.h>])
 AC_CHECK_DECLS([strlcat, strlcpy, strnlen])