Hotpatch CXXFLAGS to be the same as CFLAGS if CXXFLAGS is not set. (#23568)
Summary:
This fixes build regression caused by https://github.com/pytorch/pytorch/issues/23528 because we used to let CXXFLAGS equal CFLAGS.
cc suo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23568
Differential Revision: D16568820
Pulled By: suo
fbshipit-source-id: 64a0dc923c08ac1751224f42bc4ccdc707341762
