Skip building docs during make install on PR builds

[staticfloat]

Since we already have a doctest build, I think it would save us a nice chunk of time if we could skip the docs build that occurs during make install (which happens when we do make binary-dist). Since I think we want the docs in the uploaded tarballs, we'll want to build these when running on master (or release-*) but on PR builds, we can shave off something like 7% (~1 minute) of our build time by skipping this, and this happens for every build configuration.