Make the default integer size to be `MRB_INT64` on 64bit systems; close
Unless `MRB_NAN_BOXING` is specified.
MODIFY
include/mrbconf.h
include/mrbconf.h
/* add -DMRB_INT64 to use 64bit integer for mrb_int; conflict with MRB_INT16 */
//#define MRB_INT64
+/* if no specific integer type is chosen */
+#if !defined(MRB_INT16) && !defined(MRB_INT32) && !defined(MRB_INT64)
+# if defined(MRB_64BIT) && !defined(MRB_NAN_BOXING)
+/* Use 64bit integers on 64bit architecture (without MRB_NAN_BOXING) */
+# define MRB_INT64
+# else
+/* Otherwise use 32bit integers */
+# define MRB_INT32
+# endif
+#endif
+
/* represent mrb_value in boxed double; conflict with MRB_USE_FLOAT */
//#define MRB_NAN_BOXING

Make the default integer size to be `MRB_INT64` on 64bit systems; close #3800

Unless `MRB_NAN_BOXING` is specified.