USE CGYWIN_V15_WIN32API as macro to select api for cygwin

The previous macro was confusing to some, and did not include "cygwin" in
its name. The updated name more clearly expresses a choice of the
win32api implementation that shipped with version 1.5 of cygwin.
Signed-off-by: NMark Levedahl <mlevedahl@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1086,7 +1086,7 @@ ifeq ($(uname_O),Cygwin)
 		NO_SYMLINK_HEAD = YesPlease
 		NO_IPV6 = YesPlease
 		OLD_ICONV = UnfortunatelyYes
-		V15_MINGW_HEADERS = YesPlease
+		CYGWIN_V15_WIN32API = YesPlease
 	endif
 	NO_THREAD_SAFE_PREAD = YesPlease
 	NEEDS_LIBICONV = YesPlease
@@ -1894,8 +1894,8 @@ ifdef NO_REGEX
 	COMPAT_CFLAGS += -Icompat/regex
 	COMPAT_OBJS += compat/regex/regex.o
 endif
-ifdef V15_MINGW_HEADERS
-	COMPAT_CFLAGS += -DV15_MINGW_HEADERS
+ifdef CYGWIN_V15_WIN32API
+	COMPAT_CFLAGS += -DCYGWIN_V15_WIN32API
 endif
 
 ifdef USE_NED_ALLOCATOR

compat/cygwin.c

 #define WIN32_LEAN_AND_MEAN
-#ifdef V15_MINGW_HEADERS
+#ifdef CYGWIN_V15_WIN32API
 #include "../git-compat-util.h"
 #include "win32.h"
 #else