diff --git a/Makefile b/Makefile
index d337905..6d057e4 100644
--- a/Makefile
+++ b/Makefile
@@ -37,7 +37,7 @@
 GOOGLE_PERFTOOLS_LDFLAGS=
 endif
 
-CFLAGS = -c -I. -I./include $(PORT_CFLAGS) $(PLATFORM_CFLAGS) $(OPT) $(SNAPPY_CFLAGS)
+CFLAGS += -c -I. -I./include $(PORT_CFLAGS) $(PLATFORM_CFLAGS) $(OPT) $(SNAPPY_CFLAGS)
 
 LDFLAGS += $(PLATFORM_LDFLAGS) $(SNAPPY_LDFLAGS) $(GOOGLE_PERFTOOLS_LDFLAGS)