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

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.

This back-patches commit 9ff47ea41 into out-of-support branches,
pursuant to newly-established project policy. The point is to
suppress scary-looking warnings so that people building these
branches needn't expend brain cells verifying that it's safe
to ignore the warnings.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c16a78147ca104602d18c8cc19ff3ed98606717c

Modified Files
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configure | 42 ++++++++++++++++++++++++++++++++++++++++++
configure.in | 8 ++++++++
2 files changed, 50 insertions(+)