Fix parallel make

Flock does not have queueing, so it's very possible that multiple threads
will cause a timeout of one of the threads.
Further, the createrepos only needs to be run once to pull in all SRPMs or
all RPMs.

This change creates a queue which will run createrepos a minimum number of
times, blocking make until createrepos has been run for that particular
(S)RPM.
Merge pull request #526 from BobBall/parallel-fix

Fix parallel make