Fix mingw detection on Windows 7 (NT-6.1)

Hopefully also for other non-XP Windows build environments.

Change-Id: I7f856dc4847c4ca9197b1935b7a9b9071b46c70a
Signed-off-by: Miklós Márton <martonmiklosqdev@gmail.com>
Reviewed-on: https://review.coreboot.org/23865
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
2 files changed