Fix build.

piwik.php

@@ -15,7 +15,10 @@ use Piwik\Tracker;
 $GLOBALS['PIWIK_TRACKER_DEBUG_FORCE_SCHEDULED_TASKS'] = false;
 define('PIWIK_ENABLE_TRACKING', true);
 
-define('PIWIK_DOCUMENT_ROOT', dirname(__FILE__) == '/' ? '' : dirname(__FILE__));
+if (!defined(PIWIK_DOCUMENT_ROOT)) {
+    define('PIWIK_DOCUMENT_ROOT', dirname(__FILE__) == '/' ? '' : dirname(__FILE__));
+}
+
 if (file_exists(PIWIK_DOCUMENT_ROOT . '/bootstrap.php')) {
     require_once PIWIK_DOCUMENT_ROOT . '/bootstrap.php';
 }