introduce an option to use Intel Thread Building Blocks

This commit introduces a new command line option, --tbb, that enables the use of
Intel Thread Building Blocks' tbb::concurrent_unordered_set for the
multithreaded checker. For our current work horse, long-running2, this appears
to make no discernible difference in its runtime characteristics. However, I've
still committed this because it's relatively low overhead and I wonder if it may
prove useful on future models.