Summary:
Update CMakeLists.txt to check the current platform, and to set compiler flags
correctly.  It now uses flags for Microsoft Visual Studio on Windows, and flags
for gcc or clang on all other platforms.  Previously it unconditionally used
MSVC flags.

Reviewed By: meyering

Differential Revision: D6710435

fbshipit-source-id: dbae3097bcadf1ee4a25879dd7770603387c0e4d