diff --git a/tools/gpio/Makefile b/tools/gpio/Makefile
index 240eda014b371b4e1c63f607e6853d387f475392..6ecdd10678260f278475787af8d7b1e61e970bae 100644
--- a/tools/gpio/Makefile
+++ b/tools/gpio/Makefile
@@ -12,7 +12,7 @@ endif
 # (this improves performance and avoids hard-to-debug behaviour);
 MAKEFLAGS += -r
 
-CFLAGS += -O2 -Wall -g -D_GNU_SOURCE -I$(OUTPUT)include
+override CFLAGS += -O2 -Wall -g -D_GNU_SOURCE -I$(OUTPUT)include
 
 ALL_TARGETS := lsgpio gpio-hammer gpio-event-mon
 ALL_PROGRAMS := $(patsubst %,$(OUTPUT)%,$(ALL_TARGETS))