Clean up build warnings of plperl with clang-12+

[Michael Paquier]

Hi all,

This is a follow-up of the discussion that happened here:
https://www.postgresql.org/message-id/yyojyo475dsqy...@paquier.xyz

Recently, we are getting warnings in the build of plperl when using at
least clang-12 because of their new flag -Wcompound-token-split-by-macro,
all the warnings coming from Perl itself.

Upstream has posted a fix about that but it will take time before this
propagates across the buildfarm (five~ animals are now impacted?):
https://www.nntp.perl.org/group/perl.perl5.changes/2021/07/msg57758.html

dangomushi is impacted by that, and I'd like to stick an -Werror in it
as it is proving to be good in detecting incorrect placeholders.

Tom has suggested that we could add -Wno-compound-token-split-by-macro
to take care of the issue on our side, and attached is a patch to do
so.

Any objections?  I'd like to get this back-patched.

Thanks,
--
Michael

diff --git a/configure b/configure
index 4ffefe4655..6856ec649d 100755
--- a/configure
+++ b/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 ${CC} supports -Wformat-truncation, for NOT_THE_CFLAGS" >&5
diff --git a/configure.ac b/configure.ac
index 44ee3ebe2f..05a00b3e14 100644
--- a/configure.ac
+++ b/configure.ac
@@ -530,6 +530,14 @@ if test "$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])

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