Support Library Requirements for GCC 7.1

[Paul Smith]

On Tue, 2017-05-02 at 18:17 -0500, Joel Sherrill wrote:
> With gcc 6.3.0, we have this in our build recipe:
> 
> %define mpfr_version   2.4.2
> %define mpc_version    0.8.1
> %define gmp_version    4.3.2

Best thing to do is look at the contrib/download_prerequisites script
that comes with the GCC source code.

The 7.1 version says:

    gmp='gmp-6.1.0.tar.bz2'
    mpfr='mpfr-3.1.4.tar.bz2'
    mpc='mpc-1.0.3.tar.gz'
    isl='isl-0.16.1.tar.bz2'

In general, it's better to just run that script, if possible.