MAKE_PARALLEL is now compatible with jobserver mode.  To make a
parallel build using the top level makefile, run e.g.

  make MAKE_PARALLEL=-j20 all run_hilfe

The targets will be built with -j20, but each target will be finished
before the next is started.

When running make in the build directory, MAKE_PARALLEL is not
meaningful.  Just run "make -j20" directly (and don't specify more
than one target).