Suppress clang's unhelpful gripes about -pthread switch being unused.

Considering the number of cases in which "unused" command line arguments
are silently ignored by compilers, it's fairly astonishing that anybody
thought this warning was useful; it's certainly nothing but an annoyance
when building Postgres.  One such case is that neither gcc nor clang
complain about unrecognized -Wno-foo switches, making it more difficult
to figure out whether the switch does anything than one could wish.

Back-patch to 9.3, which is as far back as the patch applies conveniently
(we'd have to back-patch PGAC_PROG_CC_VAR_OPT to go further, and it doesn't
seem worth that).

configure

@@ -4723,6 +4723,47 @@ if test x"$pgac_cv_prog_cc_cflags__ftree_vectorize" = x"yes"; then
   CFLAGS_VECTOR="${CFLAGS_VECTOR} -ftree-vectorize"
 fi
 
+  # We want to suppress clang's unhelpful unused-command-line-argument warnings
+  # but gcc won't complain about unrecognized -Wno-foo switches, so we have to
+  # test for the positive form and if that works, add the negative form
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports -Wunused-command-line-argument" >&5
+$as_echo_n "checking whether $CC supports -Wunused-command-line-argument... " >&6; }
+if ${pgac_cv_prog_cc_cflags__Wunused_command_line_argument+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+CFLAGS="$pgac_save_CFLAGS -Wunused-command-line-argument"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_prog_cc_cflags__Wunused_command_line_argument=yes
+else
+  pgac_cv_prog_cc_cflags__Wunused_command_line_argument=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$pgac_save_CFLAGS"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_cc_cflags__Wunused_command_line_argument" >&5
+$as_echo "$pgac_cv_prog_cc_cflags__Wunused_command_line_argument" >&6; }
+if test x"$pgac_cv_prog_cc_cflags__Wunused_command_line_argument" = x"yes"; then
+  NOT_THE_CFLAGS="${NOT_THE_CFLAGS} -Wunused-command-line-argument"
+fi
+
+  if test -n "$NOT_THE_CFLAGS"; then
+    CFLAGS="$CFLAGS -Wno-unused-command-line-argument"
+  fi
 elif test "$ICC" = yes; then
   # Intel's compiler has a bug/misoptimization in checking for
   # division by NAN (NaN == 0), -mp1 fixes it, so add it to the CFLAGS.

configure.in

@@ -445,6 +445,13 @@ if test "$GCC" = yes -a "$ICC" = no; then
   # Optimization flags for specific files that benefit from vectorization
   PGAC_PROG_CC_VAR_OPT(CFLAGS_VECTOR, [-funroll-loops])
   PGAC_PROG_CC_VAR_OPT(CFLAGS_VECTOR, [-ftree-vectorize])
+  # We want to suppress clang's unhelpful unused-command-line-argument warnings
+  # but gcc won't complain about unrecognized -Wno-foo switches, so we have to
+  # test for the positive form and if that works, add the negative form
+  PGAC_PROG_CC_VAR_OPT(NOT_THE_CFLAGS, [-Wunused-command-line-argument])
+  if test -n "$NOT_THE_CFLAGS"; then
+    CFLAGS="$CFLAGS -Wno-unused-command-line-argument"
+  fi
 elif test "$ICC" = yes; then
   # Intel's compiler has a bug/misoptimization in checking for
   # division by NAN (NaN == 0), -mp1 fixes it, so add it to the CFLAGS.