proposal to enable parallel package checks using 'gnu parallel'

Hello Dan,

could you look at the following proposal to enable parallel package checks?

The request needs some additional work: 
check if gnu parallel is around and if not, run checks as usual; any help with this?

Remarks:
 * `{}` is the replacement placeholder
 * the `halt -1` option will ensure that the checks are aborted if an error occurs.


Thanks,

Jakob