Silence compiler warnings with Python 3.9

Starting with Python 3.9, the Python headers contain inline functions
that fall afoul of our -Wdeclaration-after-statement coding style. In
order to silence those warnings, I've added some GCC-specific
contortions to disable that warning for Python.h only. Clang doesn't
appear to warn about this at all; maybe it recognizes that this is an
external header file. We could also write a configure check for this if
we want to be more flexible.

(Attempts to convince upstream to change the coding style were
unsuccessful (https://bugs.python.org/issue39615).)

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Peter Eisentraut http://www.2ndQuadrant.com/
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