mstd_cstddef: add C support and macros for C++11

Author: mprse

platform/cxxsupport/mstd_cstddef

@@ -27,8 +27,12 @@
  * - - MSTD_CONSTEXPR_XX_14 macros that can be used to mark
  *     things that are valid as constexpr only for C++14 or later,
  *     permitting constexpr use where ARM C 5 would reject it.
+  * - - MSTD_CONSTEXPR_XX_11 macros that can be used to permit
+  *     constexpr alternatives for C.
  */
 
+#if __cplusplus
+
 #include <cstddef>
 
 /* Macros that can be used to mark functions and objects that are
@@ -42,6 +46,9 @@
 #define MSTD_CONSTEXPR_OBJ_14 const
 #endif
 
+#define MSTD_CONSTEXPR_FN_11 constexpr
+#define MSTD_CONSTEXPR_OBJ_11 constexpr
+
 #ifdef __CC_ARM
 
 // [expr.alignof]
@@ -72,4 +79,13 @@ using std::max_align_t;
 
 }
 
+#else // __cplusplus
+
+#define MSTD_CONSTEXPR_FN_14 inline
+#define MSTD_CONSTEXPR_OBJ_14 const
+#define MSTD_CONSTEXPR_FN_11 inline
+#define MSTD_CONSTEXPR_OBJ_11 const
+
+#endif // __cplusplus
+
 #endif // MSTD_CSTDDEF_