diff --git a/usr.bin/xo/Makefile b/usr.bin/xo/Makefile
--- a/usr.bin/xo/Makefile
+++ b/usr.bin/xo/Makefile
@@ -22,3 +22,5 @@
 SUBDIR.${MK_TESTS}+= tests
 
 .include <bsd.prog.mk>
+
+CWARNFLAGS+=	${NO_WUNUSED_BUT_SET_VARIABLE}