PHP define's do not work they way they do in C++. They are not part of a pre-processor, but instead actually evaluate like a function. Thus, the way the code is structured drupal_get_path() is running on every single page. It's better to put this "constant" in a function, and return it's value.
Also the full path to the include is not necessary. I frequently get PHP errors caused by this when using drush. I'm not sure exactly why this happens. But I know the solution is to not use the full path.
PHP Fatal error: Cannot redeclare template_preprocess_views_view_views_rss() (previously declared in /users/me/Sites/mysite/sites/all/modules/contrib/views_rss/theme/theme.inc:11) in /Users/me/Sites/mysite/sites/all/modules/contrib/views_rss/theme/theme.inc on line 167