Authcahce with memcache: Fatal error: Call to undefined function lock_acquire().

When enabling authcache and memcache, I get this error:

Call to undefined function lock_acquire() in /var/www/forum/includes/bootstrap.inc on line 935

One way to resolve it is to modify this code in authcache.inc line 66 - line 71

 // Connect to database if default database cache hander is selected (or memcached which currently requires database access to run variable_initialize)
    if ($conf['authcache_is_db'] || $memcache == 'MemCacheDrupal') {
      require_once DRUPAL_ROOT . '/includes/database/database.inc';

      require_once DRUPAL_ROOT . '/includes/lock.inc';
      db_set_active();

    }

But I know memcache has a memcache based locking included in memcache-lock.inc, I tried loading that file but it gives me fatal error for re-declaring lock_aquire().

My settings for memcache and authcache are :

 $conf['cache_backends'][] = 'sites/all/modules/contrib/memcache/memcache.inc';
 $conf['cache_backends'][] = 'sites/all/modules/contrib/authcache/authcache.inc';
 $conf['cache_default_class'] = 'MemCacheDrupal';
 $conf['cache_class_cache_page']      = 'MemCacheDrupal';