boost_thread can be remvoed from vc++ builds, but required for others

[evoskuil]

boost-thread is a dependency of boost-locale (I believe because of lib ICU) on non-Windows builds. The windows platform doesn't require lib ICU, so this dependency falls away. The windows build works without boost-thread, but I don't believe there's a way to differentiate. If not then we can just leave it is is.