diff --git a/common.mak b/common.mak
index 02c730338c..9cc735786f 100644
--- a/common.mak
+++ b/common.mak
@@ -18,7 +18,7 @@ endif
 
 ALLFFLIBS = avcodec avdevice avfilter avformat avutil postproc swscale
 
-CPPFLAGS += -DHAVE_AV_CONFIG_H -I$(BUILD_ROOT_REL) -I$(SRC_PATH)
+CPPFLAGS := -DHAVE_AV_CONFIG_H -I$(BUILD_ROOT_REL) -I$(SRC_PATH) $(CPPFLAGS)
 
 %.o: %.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) $(LIBOBJFLAGS) -c $(CC_O) $<