diff --git a/tools/testing/selftests/mqueue/Makefile b/tools/testing/selftests/mqueue/Makefile
index 0e3b41eb85cde2cd553bda36ac0b09f43553ceb8..ca8327f020b95fbb7c05d51ced70f1e1d8e48175 100644
--- a/tools/testing/selftests/mqueue/Makefile
+++ b/tools/testing/selftests/mqueue/Makefile
@@ -1,4 +1,4 @@
-CFLAGS = -O2
+CFLAGS += -O2
 
 all:
 	$(CC) $(CFLAGS) mq_open_tests.c -o mq_open_tests -lrt