See https://github.com/akunzai/joomla-external-login
The External Login project is the continuation of the Authentication Manager project, originally developed for Joomla! 1.5, and now compatible with Joomla! 3.x. Documentation can be found in the wiki pages.
Simply execute composer install and composer build on the command line and get the built packages in the build directory.
$ composer install
$ composer build- Joomla! 3.x
You can find the version for Joomla! 2.5 here.