MSYS2 Windows support

Is there any plan (or is it worth it) to port the library to msys2?

I have tried using the following libraries with msys2/mingw64:
- boost
- gflags
- glog
- libevent
- lz4
- lzma
- snappy
- zlib

It fails while configuring itself in _FOLLY_CPP_ATOMIC_BUILTIN_ dependency. I tried removing this whole line, as this very example compiles in a separate file.

But after doing that it starts failing with lots of the following tests, probably because it is compiling as if the operating system is a Unix.

[CopyOfCMakeCache.txt](https://github.com/facebook/folly/files/2937093/CopyOfCMakeCache.txt)