multithreading hopefully working on OSX (based on patch by TurfIt)

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6642 8aca7d54-2c30-db11-9de9-000461428c89
limits for map height and roughness based on combination of the two

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6643 8aca7d54-2c30-db11-9de9-000461428c89