Thanks torabora@ for yet again seemingly obvious feature request
which strangely managed to evade implementation before.

On an afterthought, mass builds would suggest too much coffee
instead of a progress indicator -- so implemented the latter.

NB: the actual downstream-make-calling rule would expand the "naive"
$(shell date) too early: the rule is evaluated before starting its
execution, and as it's the time consuming one the shell evaluation
was in need, not make's.  The result is less generally available
(needs to be double quoted and won't work inside e.g. awk programs)
but way more precise.