recursive make can break parallel make -j

Recursive make is dangerously stupid.  There are better tools, but not everyone has realized.

In a system using recursive make, two rules invoke recursive make for different rulesof the same directory.
Unfortunately, these rukes are not parallelizable.  The nonrecursive makefile knows this, but the separate recursive make invocations do not.

I.e. recursive make can break parallel make -j.