testing consistency

[Joe Buck]

> I think it is reasonable to consider comparing as part of the checking
> process.  Right now, make doesn't imply make check.  I can envision a
> change in make, so that check is in fact implied.  make && make
> install then has neat interesting semantics.

The full "make check" takes too long, so it's not a bad idea to enforce
it.  However, if a three-stage build has already been done, make compare
is trivial, so we might as well do it.