diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -718,6 +718,17 @@
 #define	__POSIX_VISIBLE		198808
 #define	__ISO_C_VISIBLE		0
 #endif /* _POSIX_C_SOURCE */
+/*
+ * Both glibc and OpenBSD enable c11 features when _ISOC11_SOURCE is defined, or
+ * when compiling with -stdc=c11. A strict reading of the standard would suggest
+ * doing it only for the former. However, a strict reading also requires C99
+ * mode only, so building with C11 is already undefined. Follow glibc's and
+ * OpenBSD's lead for this non-standard configuration for maximum compatibility.
+ */
+#if _ISOC11_SOURCE || (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L)
+#undef __ISO_C_VISIBLE
+#define __ISO_C_VISIBLE		2011
+#endif
 #else
 /*-
  * Deal with _ANSI_SOURCE: