Set MKLDNN (default) build variables in CMakeLists.txt, not in Python build scripts (#22215)
Summary:
This is yet another step to disentangle Python build scripts and CMake
and improve their integration (Let CMake handle more build environment
detections, and less by our handcrafted Python scripts).
The processor detection logic also changed a bit: Instead of detecting
whether the system processor is PPC or ARM, this PR changes to detect
Intel CPUs, because this is more precise as MKL only supports Intel
CPUs. The build option `USE_MKLDNN` will also not be presented to
users on non-Intel processors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22215
Differential Revision: D16005953
Pulled By: ezyang
fbshipit-source-id: bf3f74d53609b3f835e280f63a872ff3c9352763
