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

diff --cc Makefile
Simple merge

diff --cc config.mak.uname
Simple merge

diff --cc git-compat-util.h
Simple merge

diff --cc help.c
Simple merge

diff --cc test-chmtime.c
Simple merge