add cmake option to toggle pthreads

[adammoody]

This defines a new -DENABLE_PTHREADS=[ON/OFF] option that one can use to toggle pthreads. It defaults to ON, so this lets one disable building the pthread support. That might be helpful for someone running on a system where pthreads is not installed, or more likely, one does not want to add a -lpthread to their link because they otherwise don't have a pthreads dependency.

[gonsie]

The original contents of this PR got included by #123. I've fixed up one small change, but really see d3ab8d9