As noticed by fabio, system is not needed anymore to build the game on OSX.
Tested on OSX 10.9.5 Mavericks
I think we do not need to include boost::system because we don't use anything from it, but we need to link it because boost::filesystem depends on it. I think only the build-osx change should be kept, as bootstrap is clever enough to build the boost::system lib while now excluding it from distributed includes.
I though Mac OS had a tendency to link dynamically when it could ? It isn't a big change, it more of a cleanup. If you have a good argument against committing it, I can just abandon the revision :)