Update build for Cygwin 1.[57].  Torsten Bögershausen reports that
this is fine with Cygwin 1.7 ($gmane/225824) so let's try moving it
ahead.

* rj/mingw-cygwin:
  cygwin: Remove the CYGWIN_V15_WIN32API build variable
  mingw: rename WIN32 cpp macro to GIT_WINDOWS_NATIVE