--- a/src/share/mk/bsd.sys.mk
+++ b/src/share/mk/bsd.sys.mk
@@ -29,7 +29,8 @@
 #CFLAGS+=	-pedantic
 .if defined(WARNS)
 .if ${WARNS} >= 1
-CWARNFLAGS+=	-Wsystem-headers
+# Disabled (see #664242)
+#CWARNFLAGS+=	-Wsystem-headers
 .if !defined(NO_WERROR) && (${COMPILER_TYPE} != "clang" \
     || !defined(NO_WERROR.clang))
 CWARNFLAGS+=	-Werror
