Index: sys/sys/cdefs.h
===================================================================
--- sys/sys/cdefs.h
+++ sys/sys/cdefs.h
@@ -718,6 +718,19 @@
 #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, but defining
+ * _ISOC11_SOURCE is both compatible with glibc, and easy enough to do to enable
+ * this relatively standard extension to POSIX that's not yet been standardized.
+ * The POSIX-1.2018 standard allows enabling additional interfaces when certain
+ * Feature Test Macros are defined, so define C11 interfaces when _ISOC11_SOURCE
+ * is defined.
+ */
+#if _ISOC11_SOURCE
+#undef __ISO_C_VISIBLE
+#define __ISO_C_VISIBLE		2011
+#endif
 #else
 /*-
  * Deal with _ANSI_SOURCE: