pgsql: Hide warnings from Python headers when using gcc-compatible comp

Hide warnings from Python headers when using gcc-compatible compiler.

Like commit 388e80132, use "#pragma GCC system_header" to silence
warnings appearing within the Python headers, since newer Python
versions no longer worry about some restrictions we still use like
-Wdeclaration-after-statement.

This patch improves on 388e80132 by inventing a separate wrapper
header file, allowing the pragma to be tightly scoped to just
the Python headers and not other stuff we have laying about in
plpython.h. I applied the same technique to plperl for the same
reason: the original patch suppressed warnings for a good deal
of our own code, not only the Perl headers.

Like the previous commit, back-patch to supported branches.

Peter Eisentraut and Tom Lane

Discussion: /messages/by-id/ae523163-6d2a-4b81-a875-832e48dec502@eisentraut.org

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b0115e7e20a6cbf1705d0661be09383c89c59bc1

Modified Files
--------------
src/pl/plperl/GNUmakefile | 4 +-
src/pl/plperl/meson.build | 1 +
src/pl/plperl/plperl.h | 195 +---------------------------------
src/pl/plperl/plperl_system.h | 215 ++++++++++++++++++++++++++++++++++++++
src/pl/plpython/Makefile | 3 +-
src/pl/plpython/meson.build | 1 +
src/pl/plpython/plpython.h | 23 +---
src/pl/plpython/plpython_system.h | 53 ++++++++++
8 files changed, 280 insertions(+), 215 deletions(-)