CIviCRM class API should check if constant is defined before defining it

This is kind of an odd use case but we are calling the class api in tests and it is hitting an issue where 'CIVICRM_SETTINGS_PATH' is already defined

There is precedent elsewhere for putting that define in an IF so I'm going to add a patch to do that here too.

if (!defined('CIVICRM_SETTINGS_PATH'))

{ define('CIVICRM_SETTINGS_PATH', $config['conf_path'] . '/civicrm.settings.php'); }