Don't use -mt when linking with boost_serialization
Don't use -mt when linking with boost_serialization

I don't remember what the issue was there, but both Ubuntu and Fedora are
shipping libboost_serialization and we don't use boost in a multi-threaded way,
so this should be fine.

RaphaelRochet/easystroke,thjaeger/easystroke,thjaeger/easystroke,RaphaelRochet/easystroke