Better fix for Autoload when running from get.php

I believe this bit:
define('BP', dirname(dirname(dirname(dirname(dirname(__FILE__))))));
does not work if the Autoload.php file is placed using modman. If you manually copy the file to docroot/app/code/local/Varien/Autoload.php I believe it works. 

The dirname bit will traverse up the real path as opposed to the symlinked path. I also used an internal variable instead of BP because later on BP may be redfined which throws a notice.