Link to pthreads on non-Apple Unices.
Link to pthreads on non-Apple Unices.

This seems to be necessary in some but not all cases, still haven't
pinned down when this changed (upgrade to Mongo 2.0 triggered it here).

Change-Id: Ifd9ceac703905bac10d8a5a315c3f5984532ccc1

OpenChemistry/mongochem,OpenChemistry/mongochem,OpenChemistry/mongochem