Libraries API

It appears that both Libraries API and Variables API provide a function to retrieve the path to third-party library code installed under sites/all/libraries/.

From libraries.module, lines 9-22

<?php
/**
* Helper function to build paths to libraries.
*
* @param $name
* The machine name of a library to return the path for.
* @param $base_path
* Whether to prefix the resulting path with base_path().
*
* @return
* The path to the specified library.
*