Consistent use of USE_MINGW

Change-Id: I05e212e5a99639d0196006b9c2ec35072c54f399
diff --git a/tools/zipalign/Android.mk b/tools/zipalign/Android.mk
index 708c8bf..49f9043 100644
--- a/tools/zipalign/Android.mk
+++ b/tools/zipalign/Android.mk
@@ -24,7 +24,7 @@
 LOCAL_LDLIBS += -lrt
 endif
 
-ifneq ($(strip $(USE_MINGW)),)
+ifdef USE_MINGW
 LOCAL_STATIC_LIBRARIES += libz
 else
 LOCAL_LDLIBS += -lz