Something I wondered from watching that talk, rather than trying to
make ghc run concurrently internally, can we just have --make, when
faced with multiple possibilities, pick the first one without a
'ModuleName.working' file, create such a working file, and then go to?

Then you can run 'ghc --make X.hs &; ghc --make X.hs &; ...'.

In fact, isn't that what make -j already does?  I could try it with
the old style 'ghc -M' and pure makefile, but it turns out to be a lot
of work to figure out what packages to include and tangle out the
right .o files and whatnot, work that --make does for me.