Simply disallow <stdnoreturn.h> to be used in combination with C++.

There is no way one could possibly use this header file in combination
with C++ code. The problem is that in C11 the `noreturn' macro expands
to the `_Noreturn' function specifier, while in C++11 the `noreturn'
keyword is an attribute.

So in C11 you have to write:

	noreturn void exit(int status);

While in C++11 you have to write:

	[[noreturn]] void exit(int status);

It is impossible to #define noreturn for C++ in such a way that it
allows both conventions.

By intentionally breaking this header this way, we prevent people from
using this header in their C++<11 sources.

include/stdnoreturn.h

@@ -26,11 +26,13 @@
  * $FreeBSD$
  */
 
-#ifndef noreturn
-
-#if !defined(__cplusplus) || __cplusplus < 201103L
-#include <sys/cdefs.h>
-#define	noreturn		_Noreturn
+#ifdef __cplusplus
+#error "<stdnoreturn.h> cannot be used in combination with C++11."
 #endif
 
+#ifndef noreturn
+
+#include <sys/cdefs.h>
+#define	noreturn		_Noreturn
+
 #endif /* !noreturn */