How to use a $config setting in a pre_system hook [SOLVED]

[eluser]JoostV[/eluser]
I would like to use a config setting in a pre_system hook. Can anybody tell me if and how this can be done?

Specifically, I would like to set some URL-related constants like C_ROOT_URL in my pre_system bootstrap hook class, using config.php's

Code:

$config['base_url']

I already set stuff in this class like

Code:

// Set environment we are currently in
$rootpath = dirname(realpath(BASEPATH)) . DIRECTORY_SEPARATOR;
if (stristr($rootpath, 'O:\htdocs')) {
    define('C_ENVIRONMENT', 'development');
}
elseif (stristr($rootpath, '/home/testuser/domains/thisismytestdomain.com')) {
    define('C_ENVIRONMENT', 'staging');
}
else {
    define('C_ENVIRONMENT', 'production');
}