Makefile change to restart bootstraps

[Richard Kenner]

    I can see it's advisable to do it when testing GCC, but remember that
    these rules are also for people who're going to build GCC to use.

Right.

    They used to get quite frustrated when `make bootstrap' re-started the
    build pretty much from scratch, and it would be quite annoying to have
    the bootstrap restarted just because you tweak something in a config.h
    file.

But remember that machines are much faster than when people "used to"
compain!  The bootstrap shouldn't *need* to be "restarted". I'd argue
quite the opposite: if a bootstrap failed for some reason, it's
necessary to understand why and fix it so that it can run completely
through.  Otherwise, how can you have confidence in the result?

And *certainly* if somebody other than a developer "teawked" something in
config.h, I'd want the bootstrap to *definitely* restart from the beginning
since he can't be sure how much it affects.

In fact the "normal user" argument makes it even stronger that we should not
have this sort of restarting bootstrap since it's much harder for normal
users to know they have to take steps to start over than for dvelopers.