Postpone LLVM-related uses of AC_CHECK_DECLS.

Calling AC_CHECK_DECLS before we've finished setting up the compiler's
CFLAGS seems like a pretty risky proposition, especially now that the
first use of that macro will result in a test to see whether the compiler
gives warning or error for undeclared built-in functions.  That answer
could very easily get changed later than where PGAC_LLVM_SUPPORT is
called; furthermore, it's hardly unlikely that flags such as -D_GNU_SOURCE
could change visibility of declarations.  Hence, be a little less cavalier
about where to do LLVM-related tests.  This results in v11 and HEAD doing
the warning-or-error check at the same place in the script as older
branches are doing it, which seems like a good thing.

Per further thought about commits 0b59b0e8 and 16fbac39.

config/llvm.m4

 # config/llvm.m4
 
 # PGAC_LLVM_SUPPORT
-# ---------------
+# -----------------
 #
 # Look for the LLVM installation, check that it's new enough, set the
 # corresponding LLVM_{CFLAGS,CXXFLAGS,BINPATH} and LDFLAGS
-# variables. Also verifies that CLANG is available, to transform C
+# variables. Also verify that CLANG is available, to transform C
 # into bitcode.
 #
 AC_DEFUN([PGAC_LLVM_SUPPORT],
@@ -91,14 +91,6 @@ AC_DEFUN([PGAC_LLVM_SUPPORT],
 
   LLVM_BINPATH=`$LLVM_CONFIG --bindir`
 
-  # Check which functionality is present
-  SAVE_CPPFLAGS="$CPPFLAGS"
-  CPPFLAGS="$CPPFLAGS $LLVM_CPPFLAGS"
-  AC_CHECK_DECLS([LLVMOrcGetSymbolAddressIn], [], [], [[#include <llvm-c/OrcBindings.h>]])
-  AC_CHECK_DECLS([LLVMGetHostCPUName, LLVMGetHostCPUFeatures], [], [], [[#include <llvm-c/TargetMachine.h>]])
-  AC_CHECK_DECLS([LLVMCreateGDBRegistrationListener, LLVMCreatePerfJITEventListener], [], [], [[#include <llvm-c/ExecutionEngine.h>]])
-  CPPFLAGS="$SAVE_CPPFLAGS"
-
   # LLVM_CONFIG, CLANG are already output via AC_ARG_VAR
   AC_SUBST(LLVM_LIBS)
   AC_SUBST(LLVM_CPPFLAGS)
@@ -107,3 +99,22 @@ AC_DEFUN([PGAC_LLVM_SUPPORT],
   AC_SUBST(LLVM_BINPATH)
 
 ])# PGAC_LLVM_SUPPORT
+
+
+# PGAC_CHECK_LLVM_FUNCTIONS
+# -------------------------
+#
+# Check presence of some optional LLVM functions.
+# (This shouldn't happen until we're ready to run AC_CHECK_DECLS tests;
+# because PGAC_LLVM_SUPPORT runs very early, it's not an appropriate place.)
+#
+AC_DEFUN([PGAC_CHECK_LLVM_FUNCTIONS],
+[
+  # Check which functionality is present
+  SAVE_CPPFLAGS="$CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $LLVM_CPPFLAGS"
+  AC_CHECK_DECLS([LLVMOrcGetSymbolAddressIn], [], [], [[#include <llvm-c/OrcBindings.h>]])
+  AC_CHECK_DECLS([LLVMGetHostCPUName, LLVMGetHostCPUFeatures], [], [], [[#include <llvm-c/TargetMachine.h>]])
+  AC_CHECK_DECLS([LLVMCreateGDBRegistrationListener, LLVMCreatePerfJITEventListener], [], [], [[#include <llvm-c/ExecutionEngine.h>]])
+  CPPFLAGS="$SAVE_CPPFLAGS"
+])# PGAC_CHECK_LLVM_FUNCTIONS

configure.in

@@ -1832,6 +1832,10 @@ if test "$with_icu" = yes; then
   CPPFLAGS=$ac_save_CPPFLAGS
 fi
 
+if test "$with_llvm" = yes; then
+  PGAC_CHECK_LLVM_FUNCTIONS()
+fi
+
 # Lastly, restore full LIBS list and check for readline/libedit symbols
 LIBS="$LIBS_including_readline"