Fix to defined `MRB_SYMBOL_BITSIZE` and `MRB_SYMBOL_MAX` always; ref
@@ -160,6 +160,10 @@ typedef void mrb_value;
#ifndef mrb_bool
#define mrb_bool(o) (mrb_type(o) != MRB_TT_FALSE)
#endif
+#if !defined(MRB_SYMBOL_BITSIZE)
+#define MRB_SYMBOL_BITSIZE (sizeof(mrb_sym) * CHAR_BIT)
+#define MRB_SYMBOL_MAX UINT32_MAX
+#endif
#ifndef MRB_WITHOUT_FLOAT
#define mrb_float_p(o) (mrb_type(o) == MRB_TT_FLOAT)
#endif
