diff --git a/share/mk/sys.mk b/share/mk/sys.mk
index 0d37bdc6549d..68d62847a147 100644
--- a/share/mk/sys.mk
+++ b/share/mk/sys.mk
@@ -41,7 +41,7 @@ CFLAGS		?=	-O -pipe
 .endif
 
 CXX		?=	c++
-CXXFLAGS	?=	${CFLAGS:N-std=*:N-Wnested-externs}
+CXXFLAGS	?=	${CFLAGS:N-std=*:N-Wnested-externs:N-W*-prototypes}
 
 CPP		?=	cpp