diff --git a/Makefile b/Makefile
index bddcb86..1bb0283 100644
--- a/Makefile
+++ b/Makefile
@@ -530,7 +530,7 @@
 include $(srctree)/arch/$(ARCH)/Makefile
 
 # arch Makefile may override CC so keep this after arch Makefile is included
-NOSTDINC_FLAGS := -nostdinc -isystem $(shell $(CC) -print-file-name=include)
+NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 CHECKFLAGS     += $(NOSTDINC_FLAGS)
 
 # warn about C99 declaration after statement