let's say you have a WP-Q2A integration. if you are developing a WP plugin then:
first require qa-base.php:
require_once '/PATH/TO/qa-include/qa-base.php';
and if you need more inc files:
require_once QA_INCLUDE_DIR.'ANY-FILE.php';
it will be using external codes, it is simple and flexible. but ofcourse users will need to config it's path, there is no scaping that.
if you are developing a Q2A plugin and want to use WP functions:
require_once(
'/
wp-load.php');
require_once( '/wp-includes/classes.php');
require_once( '/wp-includes/functions.php');
it should do the job, if you want to integrate two WP and Q2A plugins also include:
require_once( '/wp-includes/
plugin
.php');
What are you developing any way?