xfce-do-release: Warn about unmodified configure.ac.in
Hopefully this will prevent releasing without changing the version number.
It happened to me with xfce4-verve-plugin 2.0.2. I stopped before uploading the archive but I commited the tag, so I still had to make a 2.0.3 just to change the version number :/
Edited by Gaël Bonithon