Add version check to vendor configuration.

This is something what most Linux distributions will want to disable, so
give it to place where this customisation should happen.

libraries/config.default.php

@@ -499,7 +499,7 @@ $cfg['AjaxEnable'] = true;
  *
  * @global boolean $cfg['VersionCheck']
  */
-$cfg['VersionCheck'] = true;
+$cfg['VersionCheck'] = VERSION_CHECK_DEFAULT;
 
 /**
  * maximum number of db's displayed in left frame and database list

libraries/vendor_config.php

@@ -54,4 +54,9 @@ define('CUSTOM_HEADER_FILE', CONFIG_DIR . 'config.header.inc.php');
  * Filename of custom footer file.
  */
 define('CUSTOM_FOOTER_FILE', CONFIG_DIR . 'config.footer.inc.php');
+
+/**
+ * Default value for check for version upgrades.
+ */
+define('VERSION_CHECK_DEFAULT', true);
 ?>