said, if nothing needs to be remade, the effect of recursive make is to simply
run make several times - one for each sub-make - each of which do nothing more
than determine that nothing needs to be done.
No, this is my point (I think). The top-level make, will not
just “determine that nothing needs to be done”. It will run the subdir make,
that one will indeed determine that nothing needs to be done. But then, the
top make, will have to remake all the targets, that depended on the “subsystem”
target – because the subsystem target was “remade”.