Build: Fix use of MAKE_PARALLEL
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).