Summary:
It looks like the various CMAKE_CXX_FLAGS_* are simply ineffective, and I'm not
sure why, setting CMAKE_CXX_FLAGS does work though, and CMake appears to add
some release/debug flags to it when generating Ninja files.

Reviewed By: fanzeyi

Differential Revision: D27862117

fbshipit-source-id: a89f6182b5bae9a087f8fcfd4c5f9526f91e2adf