Fix Python.h discovery logic on some MacOS platforms (#51586)
Summary:
On all non-Windows platforms we should use 'posix_prefix' schema to discover location of Python.h header
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51586
Reviewed By: ezyang
Differential Revision: D26208684
Pulled By: malfet
fbshipit-source-id: bafa6d79de42231629960c642d535f1fcf7a427f
