Makefile change to restart bootstraps

[Richard Kenner]

    The new `make bootstrap' continues from the point where it was
    interrupted.  The main advantage is that it's restartable.  

This is the point: I don't see *any* case where that commonly needed. All
the cases you mention are where it would be a *bad idea* to restart it that
way since the previous files won't be recompiled with the changed compiler:
this is a recipe to having undetected problems.

Besides, it's always been possible to restart a bootstrap in the middle if
you wanted to: that's what "make bootstrap2" and "make bootstrap3" are for.