Faster makefile to build batteries (and doc) in parallel

new targets:

make fast -jXX
make fast-clean
make fast-doc -jXX
etc

Still, make fast rebuilds too many targets because:

- it regenerates the ml(i) files from templates (easy to avoid using a
  'make prepare' stage, for instance)
- cmi files are build both from ml and mli rules which makes some cmi
  more recent than the cmo that depends on them

The build is fast enough I didn't bother fixing this.