The present 1.19 installer detects already whether or not extensions are present in $IP/extensions during the installation and offers the user a menu to select and enable these pre-installation extensions.
However, some extensions such as Extension:OpenID and LiquidThreads require database modifications by executing special installation/database-upgrade code in the extensions. One has to start this manually _after_ the MediaWiki installation has finished (run $IP/maintenance/php update.php ).
In my view this should automatically be done by the installer in a special post-installation phase; as explained, this post-phase is only needed if the installer detected any extension and the installing user has selected at least one for installation.
Version: 1.20.x
Severity: normal
URL: http://www.mediawiki.org/wiki/Special:Code/MediaWiki/85021#c17706