mariadb55: force enabling cond_push

Optimization for the following SQL requries cond_push:

  SELECT * FROM ...
	   WHERE MATCH(...) AGAINST(..) AND
		 int_column = int_value
	   ORDER BY ... LIMIT ...;

Should we use
  SET GLOBAL optimizer_switch='engine_condition_pushdown=on';
instead to enable cond_push?

MariaDB 5.5 disables cond_push by default. We can enable it by the
above "SET" command or this change.

MySQL doesn't disable cond_push...

fixes #1315

Reported by Kazuhiko Shiozaki. Thanks!!!

ha_mroonga.cc

@@ -1974,6 +1974,9 @@ ulonglong ha_mroonga::storage_table_flags() const
     HA_CAN_GEOMETRY |
     HA_CAN_RTREEKEYS;
     //HA_HAS_RECORDS;
+#ifdef HA_MUST_USE_TABLE_CONDITION_PUSHDOWN
+  flags |= HA_MUST_USE_TABLE_CONDITION_PUSHDOWN;
+#endif
   DBUG_RETURN(flags);
 }