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>
diff --git a/Makefile b/Makefile
index 943d88d..746d8d6 100644
--- a/Makefile
+++ b/Makefile
@@ -93,10 +93,11 @@
 
 # HOST_OS is only used to work around local toolchain issues.
 HOST_OS ?= $(shell uname)
-ifeq ($(HOST_OS), MINGW32_NT-5.1)
+ifeq ($(findstring MINGW, $(HOST_OS)), MINGW)
 # Explicitly set CC = gcc on MinGW, otherwise: "cc: command not found".
 CC = gcc
 endif
+
 ifneq ($(HOST_OS), SunOS)
 STRIP_ARGS = -s
 endif