OptiTrackNatNet: link against pthread on Unix platforms
OptiTrackNatNet: link against pthread on Unix platforms

It looks like pthread is used in some function templates from
boost::asio that are used in this plugin.

Former-commit-id: f46beac3d51b0663288b5bdb5a6cfa853505e626
hdeling/sofa,hdeling/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa