Clean up build warnings of plperl with clang-12+

Hi all,

This is a follow-up of the discussion that happened here:
https://www.postgresql.org/message-id/YYoJYO475dsqYhta@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