wchar.h: only include other headers if _FORTIFY_SOURCE is enabled

unexpectedly including other headers can cause problems with
sensitive/brittle code, particularly with alternative compilers
(clang) which are already operating on the margins of what's
supported/expected by some projects.

having a way to almost entirely short-circuit these headers (by
disabling _FORTIFY_SOURCE) is therefore important.

--- a/include/wchar.h
+++ b/include/wchar.h
@@ -17,6 +17,13 @@
 #ifndef _FORTIFY_WCHAR_H
 #define _FORTIFY_WCHAR_H
 
+#if !defined(__cplusplus) && !defined(__clang__)
+__extension__
+#endif
+#include_next <wchar.h>
+
+#if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0
+
 #if !defined(__cplusplus) && !defined(__clang__)
 __extension__
 #endif
@@ -32,9 +39,7 @@ __extension__
 #if !defined(__cplusplus) && !defined(__clang__)
 __extension__
 #endif
-#include_next <wchar.h>
 
-#if defined(_FORTIFY_SOURCE) && _FORTIFY_SOURCE > 0 && defined(__OPTIMIZE__) && __OPTIMIZE__ > 0
 #include "fortify-headers.h"
 
 #ifdef __cplusplus