[configury patch] don't add 'mpfr' to --with-mpfr-dir

[Paul Brook]

> > > Also, the way --with-mpfr-dir worked made it impossible to use a
> > > standalone mpfr
> > > that wasn't installed, effectively rendering that option useless.
> >
> > Useless at least for that case, agreed.  I'm not sure which variant (gmp
> > --with-mpfr or gmp with standalone mpfr) is more common nowadays, but
> > both should work of course.
>
> I agree.  Does anybody have time to look into all permutations (GMP / MPFR
> installed / uninstalled, bundled / unbundled)?

If this turns out to get messy, or take significant amounts of effort to fix, 
I suggest removing the uninstalled option and requiring installed libraries.
Both gmp and mpfr support --prefix, so this can be done by non-root users.

Paul