[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 believe gmp --with-mpfr is unmaintained and officially obsolete/deprecated. 
We probably need to update out documentation to reflect thie.

Paul