[PATCH] Bump minimum MPFR version to 3.1.0

[Gerald Pfeifer]

On Sun, 10 Nov 2019, Janne Blomqvist wrote:
> Thanks, I'll take that as an Ok for the Fortran part. I believe I
> still need an Ok by a global or build machinery reviewer for the
> global and docs parts.

For the docs parts, and in particular a change like this, only in
the most pedantic of worlds imaginable. ;-)  

That said: okay. And thank you!

Gerald