In SVN, the folder trunk/OSX/ contains six files: binary_picker.sh, getversion.cc, osx.mk, plistgen.sh, simutrans.icns, simutrans.sh
But in the posted zip archives of the sources since version 120.2 (e.g. simutrans-src-120-2-2.zip), the OSX/ folder contains only getversion.cc, preventing compilation on OS X.
It looks like the script zipsrc.sh needs updating in order to include the other five files. (Once this is fixed and a new archive is posted, the Homebrew simutrans formula can be updated to 120.2.)
Fixed as of 120.3.