add static_assert and hide noreturn, alignas, alignof from C++

add static_assert and protect the other new C11 keyword macros
with #ifndef __cplusplus so they don't conflict with C++ keywords.

include/stdalign.h

@@ -1,6 +1,8 @@
 #ifndef _STDALIGN_H
 #define _STDALIGN_H
 
+#ifndef __cplusplus
+
 /* this whole header only works in C11 or with compiler extensions */
 #if __STDC_VERSION__ < 201112L && defined( __GNUC__)
 #define _Alignas(t) __attribute__((__aligned__(t)))
@@ -9,6 +11,9 @@
 
 #define alignas _Alignas
 #define alignof _Alignof
+
+#endif
+
 #define __alignas_is_defined 1
 #define __alignof_is_defined 1