ninja: Allow to limit the parallel build processes.

ninja: Allow to limit the parallel build processes.

When run, ninja normally runs a maximum number of processes in parallel.
By default this is the number of cores on the system plus two. In some cases this can
overheat a CPU or run a system out of memory. If run from the command line, passing a
-jN parameter will limit the number of parallel processes, but some packages embed the
execution of ninja and do not pass a -j parameter.

Using this optional procedure allows us to limit the number of parallel processes
via an environment variable, NINJAJOBS.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>