Fix for bug#23113: Different behavior on altering ENUM fields between 5.0 and 5.1

Problem: mysqld doesn't detect that enum data must be reinserted performing
'ALTER TABLE' in some cases.

Fix: reinsert data altering an enum field if enum values are changed.

sql/field.h

@@ -1853,6 +1853,8 @@ public:
   CHARSET_INFO *sort_charset(void) const { return &my_charset_bin; }
 private:
   int do_save_field_metadata(uchar *first_byte);
+  bool compare_enum_values(TYPELIB *values);
+  uint is_equal(Create_field *new_field);
 };