fix broken build after #2363
fix broken build after #2363

It isn't pretty but the install prefix has some effect during the
configure (when running cmake) and also after in the install target.
If we want to keep a pretty install prefix we would have to update every
.install file from the debian packages. I would prefer not having to do
so.

xlqian/navitia,Tisseo/navitia,kadhikari/navitia,pbougue/navitia,kinnou02/navitia,xlqian/navitia,CanalTP/navitia,Tisseo/navitia,kinnou02/navitia,pbougue/navitia,CanalTP/navitia,pbougue/navitia,patochectp/navitia,xlqian/navitia,kinnou02/navitia,patochectp/navitia,CanalTP/navitia,xlqian/navitia,CanalTP/navitia,kadhikari/navitia,Tisseo/navitia,xlqian/navitia,patochectp/navitia,kadhikari/navitia,pbougue/navitia,CanalTP/navitia,kadhikari/navitia,kinnou02/navitia,patochectp/navitia,Tisseo/navitia,Tisseo/navitia