diff --git a/share/mk/src.sys.mk b/share/mk/src.sys.mk
--- a/share/mk/src.sys.mk
+++ b/share/mk/src.sys.mk
@@ -42,7 +42,7 @@
 CFLAGS+=	-fmacro-prefix-map=${SRCTOP}=/usr/src -fdebug-prefix-map=${SRCTOP}=/usr/src
 .endif
 
-DEFAULTWARNS=	6
+DEFAULTWARNS?=	6
 
 # tempting, but bsd.compiler.mk causes problems this early
 # probably need to remove dependence on bsd.own.mk