If you use composer or upstream git repo as sub-project you cant modify config.php to your needs.
Patch allows to create an own index.php as follows:
<?php
define('ZPUSH_CONFIG', __DIR__.'/config.php');
$_SERVER['SCRIPT_FILENAME'] = realpath(__DIR__.'/../vendor/zarafa/zpush2/index.php');
chdir('../vendor/zarafa/zpush2');
include('../vendor/zarafa/zpush2/src/index.php');