--- a/src/share/mk/bsd.sys.mk
+++ b/src/share/mk/bsd.sys.mk
@@ -27,7 +27,7 @@
 #CFLAGS		+= -pedantic
 . if defined(WARNS)
 .  if ${WARNS} >= 1
-CWARNFLAGS	+=	-Wsystem-headers
+# Disabled (see #664242)
 .   if !defined(NO_WERROR)
 CWARNFLAGS	+=	-Werror
 .   endif