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