Summary:
Some compilers such as Apple Clang do not support the compiler flag `-fopenmp`. So, do not add it by default to the list of `CXX_FLAGS` that are expected for *every* compiler to support. Instead, check if the `CXX_COMPILER` specified by `CMake` supports `-fopenmp` by using `CHECK_CXX_COMPILER_FLAG`.
Closes https://github.com/facebook/folly/pull/741

Reviewed By: simpkins

Differential Revision: D6843171

Pulled By: yfeldblum

fbshipit-source-id: 671958d867400b5802d7d563036639d61d11f615