compile again if REVISION undefined

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6598 8aca7d54-2c30-db11-9de9-000461428c89
squelch compiler warnings

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6599 8aca7d54-2c30-db11-9de9-000461428c89