mingw: rename WIN32 cpp macro to GIT_WINDOWS_NATIVE

mingw: rename WIN32 cpp macro to GIT_WINDOWS_NATIVE

Throughout git, it is assumed that the WIN32 preprocessor symbol is
defined on native Windows setups (mingw and msvc) and not on Cygwin.
On Cygwin, most of the time git can pretend this is just another Unix
machine, and Windows-specific magic is generally counterproductive.

Unfortunately Cygwin *does* define the WIN32 symbol in some headers.
Best to rely on a new git-specific symbol GIT_WINDOWS_NATIVE instead,
defined as follows:

	#if defined(WIN32) && !defined(__CYGWIN__)
	# define GIT_WINDOWS_NATIVE
	#endif

After this change, it should be possible to drop the
CYGWIN_V15_WIN32API setting without any negative effect.

[rj: %s/WINDOWS_NATIVE/GIT_WINDOWS_NATIVE/g ]

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>