Merge branch 'js/msys2'

Beginning of the upstreaming process of Git for Windows effort.

* js/msys2:
  mingw: uglify (a, 0) definitions to shut up warnings
  mingw: squash another warning about a cast
  mingw: avoid warnings when casting HANDLEs to int
  mingw: avoid redefining S_* constants
  compat/winansi: support compiling with MSys2
  compat/mingw: support MSys2-based MinGW build
  nedmalloc: allow compiling with MSys2's compiler
  config.mak.uname: supporting 64-bit MSys2
  config.mak.uname: support MSys2