perl: Hide warnings inside perl.h when using gcc compatible compiler

New versions of perl trigger warnings within perl.h with our compiler
flags. At least -Wdeclaration-after-statement, -Wshadow=compatible-local are
known to be problematic.

To avoid these warnings, conditionally use #pragma GCC system_header before
including plperl.h.

Alternatively, we could add the include paths for problematic headers with
-isystem, but that is a larger hammer and is harder to search for.

A more granular alternative would be to use #pragma GCC diagnostic
push/ignored/pop, but gcc warns about unknown warnings being ignored, so every
to-be-ignored-temporarily compiler warning would require its own pg_config.h
symbol and #ifdef.

As the warnings are voluminous, it makes sense to backpatch this change. But
don't do so yet, we first want gather buildfarm coverage - it's e.g. possible
that some compiler claiming to be gcc compatible has issues with the pragma.

Author: Andres Freund <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: Discussion: https://postgr.es/m/[email protected]

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