Remove `-pthread`
Remove `-pthread`

Remove logic to stuff `-pthread` into the link flags. Besides lacking a
platform check (which is causing the Windows builds to gripe), this
isn't the preferred way to specify use of pthreads.

billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake