Fix default CXX for Windows in cpp_extensions.py (#19052)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/19017.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19052
Differential Revision: D14846702
Pulled By: soumith
fbshipit-source-id: b0e4dadaa749da0fa2d0405a1a064820d094220a
