gh-111650: Generate pyconfig.h on Windows

Prior to this change, the Py_NOGIL macro was not defined when building
C API extensions with the `--disable-gil` build on Windows. `Py_NOGIL`
was only defined as a pre-processor definition in pyproject.props, but
that is not used by C-API extensions.

This instead generates the `pyconfig.h` header on Windows as part of
the build process. For now, `Py_NOGIL` is the only macro that may
be conditionally defined in the generated file.

Author: colesbury

.gitignore

@@ -92,6 +92,7 @@ Programs/_testembed
 PC/python_nt*.h
 PC/pythonnt_rc*.h
 Modules/python.exp
+PC/pyconfig.h
 PC/*/*.exp
 PC/*/*.lib
 PC/*/*.bsc