Enable USE_NINJA in build_pytorch_libs.py if it is in PATH (#16545)
Summary:
It is required to fix the nightly conda builds.
cc zdevito ezyang soumith
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16545
Differential Revision: D13900610
Pulled By: soumith
fbshipit-source-id: 676f903a082f6f083e07245a1df38175bb82b2f7
