Allow compiling with /SUBSYSTEM:CONSOLE in Visual Studio (tested on VS 2012 and 2010) /SUBSYSTEM:WINDOWS as old default, is unaffected.

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6487 8aca7d54-2c30-db11-9de9-000461428c89