Bug#1080322: latexmk: add -parallel=N option

[Barak A. Pearlmutter]

I was thinking about it, and another way to implement latexmk
-parallel would be to have it kick out little temporary Makefiles and
invoke GNU Make on them, instead of doing its own parallel job control
and output buffering. The various make processes could be set up to
all be below a master make process, which sets up plumbing so they can
all coordinate on the level of parallelism. Maybe "I'll" try that, see
if it comes out cleaner.