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 paths pointing to build
system headers when instead we really want the host 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.

… treatment diverges even more.

…Use value algebra to simplify the logic.