Recursive `make` and `-j`

The `-j` is meant for the top-level invocation only, and the $(MAKE) pattern is standard.

The `+` or $(MAKE) "tricks" to ensure the jobserver is inherited in submake and any other subprocess are no longer needed in gmake 4.4 (from 2022) because it defaults to a FIFO instead of pipes. The author should just upgrade gmake :).

Don't do it!

https://accu.org/journals/overload/14/71/miller_2004/

Recursive Make Considered Harmful

By Peter Miller