enable parallel ghc for building git-annex

This drops a full recompile on my new 12 core laptop from 4:00 to 2:47.

It would be possible for me to use:
cabal configure --ghc-options=-j
But that also makes cabal parallelize ghc for each package it installs
to satisfy git-annex's dependencies. Since cabal is already configured
to parallize installing dependencies, that would use N^2 cpu cores,
which seems like a bad idea.

And also I'd have to remember to do it.

So I'm thinking it's better to do it by default. If a system that is
building git-annex is also busy with other things, let the scheduler
sort it out. If this impacts someone particularly badly, they can of
course avoid it with:
cabal configure --ghc-options=-j1

git-annex.cabal

@@ -280,6 +280,8 @@ Executable git-annex
   Other-Extensions: TemplateHaskell
   -- Some things don't work with the non-threaded RTS.
   GHC-Options: -threaded
+  -- Build using all CPUs
+  GHC-Options: -j
 
   -- Fully optimize for production.
   if flag(Production)