Makefile change to restart bootstraps

[Richard Kenner]

    I thought about that, but I rejected it because there may be times
    when you don't want to restart the bootstrap, but proceed from the
    stage in which you are.  But that may work, indeed.

And what are those times?  Aren't every one of those cases places where you
are asking for trouble by not recompiling everything with the new
to-be-tested compiler?