Use `_Static_assert` when C11 is available and use it in pointer size check.

include/mruby.h

@@ -429,6 +429,12 @@ void mrb_atexit(mrb_state *mrb, mrb_atexit_func func);
 #define mrb_assert_int_fit(t1,n,t2,max) ((void)0)
 #endif
 
+#if __STDC_VERSION__ >= 201112L
+#define mrb_static_assert(exp, str) _Static_assert(exp, str)
+#else
+#define mrb_static_assert(exp, str) typedef char mrb_static_assert ## __LINE__[exp]
+#endif
+
 mrb_value mrb_format(mrb_state *mrb, const char *format, ...);
 
 #if defined(__cplusplus)

src/state.c

@@ -29,7 +29,7 @@ mrb_open_allocf(mrb_allocf f, void *ud)
   mrb_state *mrb;
 
 #ifdef MRB_NAN_BOXING
-  mrb_assert(sizeof(void*) == 4);
+  mrb_static_assert(sizeof(void*) == 4, "when using NaN boxing sizeof pointer must be 4 byte");
 #endif
 
   mrb = (mrb_state *)(f)(NULL, NULL, sizeof(mrb_state), ud);