Use `MRB_SYM()` for error class retrieval; ref #5277

Note that `MRB_SYM()` is only available when `mruby/presym.h` is
included. Use `mrb_intern_lit()` otherwise.

include/mruby.h

@@ -1303,6 +1303,13 @@ MRB_API mrb_value mrb_vformat(mrb_state *mrb, const char *format, va_list ap);
    + those E_* macros requires mrb_state* variable named mrb.
    + exception objects obtained from those macros are local to mrb
 */
+#ifdef MRB_SYM
+/* mruby/presym.h is included earlier */
+# define MRB_E_SYM(sym) MRB_SYM(sym)
+#else
+# define MRB_E_SYM(sym) mrb_intern_lit(mrb, #sym)
+#endif
+
 #define E_RUNTIME_ERROR      mrb_exc_get_id(mrb, mrb_intern_lit(mrb, "RuntimeError"))
 #define E_TYPE_ERROR         mrb_exc_get_id(mrb, mrb_intern_lit(mrb, "TypeError"))
 #define E_ZERODIV_ERROR      mrb_exc_get_id(mrb, mrb_intern_lit(mrb, "ZeroDivisionError"))

include/mruby/presym/enable.h

@@ -38,4 +38,8 @@ enum mruby_presym {
   static const mrb_sym name[] = {__VA_ARGS__};                                \
   static void init_##name(mrb_state *mrb) {}
 
+/* use MRB_SYM() for E_RUNTIME_ERROR etc. */
+# undef MRB_E_SYM
+# define MRB_E_SYM(sym) MRB_SYM(sym)
+
 #endif  /* MRUBY_PRESYM_ENABLE_H */