Merge branch 'rj/mingw-cygwin'

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