Emit the C++ version when compiling pytorch from source. (#32819)
Summary:
The need for this is felt because sometimes we change a build script and change the `std=c++XX` flag, which does not get caught until the compilation has progressed for a while.
https://github.com/pytorch/pytorch/issues/31757
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32819
Differential Revision: D19697205
Pulled By: ezyang
fbshipit-source-id: b045a1d15e24c4c6007b5d1464756051d32bf911
