Cleaning up boost find_package and linkage spec

We shouldn't be attempting to link with pthread directly; Boost provides a find_package for pthread that we should use to assess whether we should link to it.

If we're going to reference Boost with find_package, we should probably reference all of the components we're using in one go.
Merge pull request #62 from leapmotion/bug-pthread

Cleaning up boost find_package and linkage spec