Clean up compilation warnings coming from PL/Perl with clang-12~

clang-12 has introduced -Wcompound-token-split-by-macro, that is causing
a large amount of warnings when building PL/Perl because of its
interactions with upstream Perl.  This commit adds one -Wno to CFLAGS at
./configure time if the flag is supported by the compiler to silence all
those warnings.

Upstream perl has fixed this issue, but it is going to take some time
before this is spread across the buildfarm, and we have noticed that
some animals would be useful with an extra -Werror to help with the
detection of incorrect placeholders (see b0cf544), dangomushi being
one.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/YYr3qYa/R3Gw+Sbg@paquier.xyz
Backpatch-through: 10

configure

@@ -6334,6 +6334,52 @@ fi
   if test -n "$NOT_THE_CFLAGS"; then
     CFLAGS="$CFLAGS -Wno-unused-command-line-argument"
   fi
+  # Remove clang 12+'s compound-token-split-by-macro, as this causes a lot
+  # of warnings when building plperl because of Perl.  Like previously, test
+  # for the positive form and add the negative form
+  NOT_THE_CFLAGS=""
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wcompound-token-split-by-macro, for NOT_THE_CFLAGS" >&5
+$as_echo_n "checking whether ${CC} supports -Wcompound-token-split-by-macro, for NOT_THE_CFLAGS... " >&6; }
+if ${pgac_cv_prog_CC_cflags__Wcompound_token_split_by_macro+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+pgac_save_CC=$CC
+CC=${CC}
+CFLAGS="${NOT_THE_CFLAGS} -Wcompound-token-split-by-macro"
+ac_save_c_werror_flag=$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__Wcompound_token_split_by_macro=yes
+else
+  pgac_cv_prog_CC_cflags__Wcompound_token_split_by_macro=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"
+CC="$pgac_save_CC"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_CC_cflags__Wcompound_token_split_by_macro" >&5
+$as_echo "$pgac_cv_prog_CC_cflags__Wcompound_token_split_by_macro" >&6; }
+if test x"$pgac_cv_prog_CC_cflags__Wcompound_token_split_by_macro" = x"yes"; then
+  NOT_THE_CFLAGS="${NOT_THE_CFLAGS} -Wcompound-token-split-by-macro"
+fi
+
+
+  if test -n "$NOT_THE_CFLAGS"; then
+    CFLAGS="$CFLAGS -Wno-compound-token-split-by-macro"
+  fi
   # Similarly disable useless truncation warnings from gcc 8+
   NOT_THE_CFLAGS=""
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wformat-truncation, for NOT_THE_CFLAGS" >&5

configure.ac

@@ -530,6 +530,14 @@ if test "$GCC" = yes -a "$ICC" = no; then
   if test -n "$NOT_THE_CFLAGS"; then
     CFLAGS="$CFLAGS -Wno-unused-command-line-argument"
   fi
+  # Remove clang 12+'s compound-token-split-by-macro, as this causes a lot
+  # of warnings when building plperl because of Perl.  Like previously, test
+  # for the positive form and add the negative form
+  NOT_THE_CFLAGS=""
+  PGAC_PROG_CC_VAR_OPT(NOT_THE_CFLAGS, [-Wcompound-token-split-by-macro])
+  if test -n "$NOT_THE_CFLAGS"; then
+    CFLAGS="$CFLAGS -Wno-compound-token-split-by-macro"
+  fi
   # Similarly disable useless truncation warnings from gcc 8+
   NOT_THE_CFLAGS=""
   PGAC_PROG_CC_VAR_OPT(NOT_THE_CFLAGS, [-Wformat-truncation])