Put a check in to be sure SM_PATH wasn't already defined. Seemed safer than just deleting the define altogether..

plugins/abook_take/setup.php

@@ -1,7 +1,9 @@
 <?php
 
 /* Path for SquirrelMail required files. */
-define('SM_PATH','../../');
+if (!defined('SM_PATH'))  {
+	define('SM_PATH','../../');
+}
 
 /* SquirrelMail required files. */
 require_once(SM_PATH . 'functions/url_parser.php');