gbuild: on WNT, target Windows XP with 7.1 SDK, and Vista with 8.x SDK

Since with the 8.x SDK it is not possible to target Windows XP anyway
according to MSDN, just set _WIN32_WINNT to _WIN32_WINNT_VISTA so that
we can at least work on cleaning up API calls deprecated in SDK 8.1.

Change-Id: Ic433caf454f57187b91ef6fdec3f3bcde3f1af79

solenv/gbuild/platform/windows.mk

@@ -21,11 +21,12 @@ gb_Helper_LIBRARY_PATH_VAR := PATH
 gb_MKTEMP := mktemp --tmpdir=$(gb_TMPDIR) gbuild.XXXXXX
 
 # define _WIN32_WINNT and WINVER will be derived from it in sdkddkver.h
+# with a 7.1 SDK target Windows XP, with 8.x SDK target Windows Vista
 # currently _WIN32_IE is defined to a higher version than would be derived
 # in sdkddkver.h from _WIN32_WINNT=0x0502 but if _WIN32_WINNT >= 0x0600
 # the derived value is sufficient
 gb_OSDEFS := \
-	-D_WIN32_WINNT=0x0502 \
+	-D_WIN32_WINNT=$(if $(filter 70,$(WINDOWS_SDK_VERSION)),0x0502,0x0600) \
 	-D_WIN32_IE=0x0700 \
 	-DWIN32 \
 	-DWNT \