I was the one who asked the original question. My purpose was so that Mercurial's test suite could specify the amount of parallelism from the Makefile. The "test" rule in the Makefile invokes "run-tests.py". This latter script itself accepts a -j option. It's possible to invoke it directly, but this misses the second part of the "test" rule in the Makefile, which executes some tests that run-tests.py can't do.