Consistent use of USE_MINGW

Change-Id: I05e212e5a99639d0196006b9c2ec35072c54f399
diff --git a/core/binary.mk b/core/binary.mk
index bda9f2e..df176f1 100644
--- a/core/binary.mk
+++ b/core/binary.mk
@@ -116,7 +116,7 @@
 # MinGW spits out warnings about -fPIC even for -fpie?!) being ignored because
 # all code is position independent, and then those warnings get promoted to
 # errors.
-ifeq ($(strip $(USE_MINGW)),)
+ifndef USE_MINGW
 ifeq ($(LOCAL_MODULE_CLASS),EXECUTABLES)
 my_cflags += -fpie
 else