Parallel make

Enable concurrent builds for RPMs and Debs.   Use make -j `<concurrency>` -l `<load limit>` to try it.   A good rule of thumb is to start with a concurrency value equal to the number of CPUs in the system.   Load limit should probably be a similar value to start with.  Don't use -l alone - when the builds start they don't do much, so the system will spawn many of them and then the load will skyrocket when they all start compiling at the same time!

Fixes issue #144