Backported mailman 2.1.23 from debian stable

helpers/make-mailman

 #!/bin/sh
 #
-#    Copyright (C) 2014  Rubén Rodríguez <ruben@trisquel.info>
+#    Copyright (C) 2014-2018  Ruben Rodriguez <ruben@trisquel.info>
 #
 #    This program is free software; you can redistribute it and/or modify
 #    it under the terms of the GNU General Public License as published by
@@ -19,6 +19,8 @@
 
 VERSION=1
 
+EXTERNAL='deb-src http://ftp.us.debian.org/debian stable main'
+
 . ./config
 
 rm debian/contrib/debianpowered.png