I just realized that you'll still run into problems when naming the next version (rc of widelands 18) correctly widelands_17.99+18rc1-maemo1 because the dot in 17.99 will be lower than the r in the proposed 17rc3 or 17release.

I see three permanent solutions:
1. Stick with the current naming scheme forever.
2. Don't package release candidates of widelands 18.
3. If you want to change your naming scheme but also want to package release candidates make sure not to call them widelands_18whatever but widelands_17whatever where "whatever" should be alphabetically higher than "rc3" respectively "release".