Index: sys/sys/cdefs.h
===================================================================
--- sys/sys/cdefs.h
+++ sys/sys/cdefs.h
@@ -718,6 +718,21 @@
 #define	__POSIX_VISIBLE		198808
 #define	__ISO_C_VISIBLE		0
 #endif /* _POSIX_C_SOURCE */
+/*
+ * glibc enables c11 features when _ISOC11_SOURCE is defined, or when compiling
+ * with -stdc=c11. It's not clear that doing this automatically is standard
+ * compliant. However, defining _ISOC11_SOURCE to enable is compatible with
+ * glibc; standards compliant; and easy enough to do to enable in a Makefile
+ * when porting. We implement this glibc extension, though it has not yet been
+ * standardized. The various POSIX-1 standard issues allow enabling additional
+ * interfaces only when certain feature test macros are defined, which is why
+ * it's not clear that doing so automatically when in C11 mode is correct,
+ * especially if that's the compiler default.
+ */
+#if _ISOC11_SOURCE
+#undef __ISO_C_VISIBLE
+#define __ISO_C_VISIBLE		2011
+#endif
 #else
 /*-
  * Deal with _ANSI_SOURCE: