Verify package integrity of downloaded prerequisites (partially fixes 61439)

[Mike Stump]

On Sep 11, 2016, at 8:35 AM, Moritz Klammler <moritz@klammler.eu> wrote:
> 
> There is a long-standing
> [bug report](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61439)
> pointing out that the `download_prerequisites` script doesn't verify the
> integrity of the packages it downloads.

I like the script.