bpo-41880: fix finding headers when cross compiling

[lopsided98]

When cross-compiling third-party extensions, get_python_inc() may be called to
return the path to Python's headers. However, it uses the sys.prefix or
sys.exec_prefix of the build Python, which returns incorrect paths when
cross-compiling (paths pointing to build system headers).

To fix this, we use the INCLUDEPY and CONFINCLUDEPY conf variables, which can
be configured to point at host Python by setting _PYTHON_SYSCONFIGDATA_NAME.
The existing behavior is maintained on non-POSIX platforms or if a prefix is
manually specified.

https://bugs.python.org/issue41880

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[lopsided98]

Equivalent fix for adopted distutils in setuptools: pypa/setuptools#2569

[iritkatriel]

Closing this PR following the closure of its b.p.o issue (https://bugs.python.org/issue41880)