Index: tools/build/mk/Makefile.boot
===================================================================
--- tools/build/mk/Makefile.boot
+++ tools/build/mk/Makefile.boot
@@ -1,6 +1,6 @@
 # $FreeBSD$
 
-CFLAGS+=	-I${WORLDTMP}/legacy/usr/include
+CFLAGS+=	-isystem ${WORLDTMP}/legacy/usr/include
 DPADD+=		${WORLDTMP}/legacy/usr/lib/libegacy.a
 LDADD+=		-legacy
 LDFLAGS+=	-L${WORLDTMP}/legacy/usr/lib