[Nix-dev] Update Julia to today's git

[Michael Raskin]

>Yes those fetchurl are my mistake and weren't used in the build anyway.
>I've however made it to use_system_mpfr and removed the useless fetchs
>(including the pcre source one which doesn't seem used).
>To do so I had to bump the mpfr version to 3.1.2.
>Patch attached. I'll be sure to setup a github fork soon.

In trunk it is better to have two versions of MPFR, I think. GCC depends
on MPFR, so having two versions of MPFR is acceptable, but altering the
default MPFR version just because of Julia is a bad idea.