Summary:
while testing https://github.com/facebook/watchman/pull/743 I
noticed that the cmake builds were picking up the installed mingw GCC
compiler rather than the MSVC compiler.  That would be fine except that
boost is built with MSVC and its generated libraries cannot be subsequently
found by a cmake gcc build that uses FindBoost.

This commit forces cmake to pick cl.exe rather than gcc.  This is probably
fine to do unconditionally on windows, but since I've only observed this
particular problem with GitHub Actions I'm keeping it constrained to that
environment for now.

Reviewed By: simpkins

Differential Revision: D17385050

fbshipit-source-id: 90bef898b138e5d4bbd28a155ed1bd468acee9de