prerequisites for installing

[Florian Weimer]

* Joakim Rosqvist:

> Humbly requesting that on the page
> https://gcc.gnu.org/install/prerequisites.html  you add something to inform
> the reader that the "download_prerequisites" mentioned there is a script in
> the contrib directory of the source code tree for gcc.

It's already mentioned here:

  <https://gcc.gnu.org/install/download.html>

Maybe the two pages should be merged?