windows build posix wrapper dependencies

The primary problem with using cygwin as a solution for a posix layer on windows is licensing, although number of dependencies and runtime costs are other things to factor in.

Winlib is a collection of headers and wrapper implementations for posix apis used by folly and hhvm
winpthreads is a fork of mingw-w64's pthreads implementation (mit licensce) patched to work properly when built and run with MSVC (14 CtP) (although do note that there are TESTS from pthreads-w32 included in the winpthreads, used to make sure library behavior is correct)

The posix layer is not done yet, and some of the implementations are currently placeholders and stubs