Index: sys/sys/cdefs.h
===================================================================
--- sys/sys/cdefs.h
+++ sys/sys/cdefs.h
@@ -718,6 +718,16 @@
 #define	__POSIX_VISIBLE		198808
 #define	__ISO_C_VISIBLE		0
 #endif /* _POSIX_C_SOURCE */
+/*
+ * glibc automatically enables c11 features when using a c11 compiler, even in
+ * strict POSIX mode. As an extension to POSIX, allow c11 features when
+ * _ISOC11_SOURCE is defined along side _POSIX_C_SOURCE to ease porting
+ * of software written under glibc.
+ */
+#if _ISOC11_SOURCE
+#undef __ISO_C_VISIBLE
+#define __ISO_C_VISIBLE		2011
+#endif
 #else
 /*-
  * Deal with _ANSI_SOURCE: