Fallback to `mrb_assert` instead of dirty trick.

include/mruby.h

@@ -432,7 +432,7 @@ void mrb_atexit(mrb_state *mrb, mrb_atexit_func func);
 #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]
+#define mrb_static_assert(exp, str) mrb_assert(exp)
 #endif
 
 mrb_value mrb_format(mrb_state *mrb, const char *format, ...);