Add torch.backends.openmp.is_available(); fix some cmake messages (#16425)
Summary:
1. add `torch.backends.openmp.is_available()`
2. Improve various `cmake` outputs
3. Fix LDFLAGS not respected by `caffe2_pybind11_state_*` targets
4. Fix `MKL` warning message, and QUIET flag.
5. Fix various typos
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16425
Differential Revision: D13903395
Pulled By: soumith
fbshipit-source-id: d15c5d46f53e1ff1c27fca2887b9d23d0bd85b4d
