diff --git a/share/mk/sys.mk b/share/mk/sys.mk
index e0527e424df6..51ec0393791a 100644
--- a/share/mk/sys.mk
+++ b/share/mk/sys.mk
@@ -66,7 +66,7 @@ CFLAGS		+=	-g
 .endif
 
 CXX		?=	c++
-CXXFLAGS	?=	${CFLAGS:N-std=*:N-Wnested-externs:N-W*-prototypes:N-Wno-pointer-sign}
+CXXFLAGS	?=	${CFLAGS:N-std=*:N-Wnested-externs:N-W*-prototypes:N-Wno-pointer-sign:N-Wold-style-definition}
 PO_CXXFLAGS	?=	${CXXFLAGS}
 
 CPP		?=	cpp