[wwwdocs] Document that gcc-8 changed the default to -std=gnu17

[Joseph Myers]

On Wed, 9 Oct 2024, Jonathan Wakely wrote:

> +<h3 id="c">C</h3>
> +  <ul>
> +    <li>New options <code>-std=c17</code>, to select support for the 2018
> +    edition of the ISO C standard, and <code>-std=gnu17</code>, for C17
> +    with GNU extensions.</li>
> +    <li>The default mode has been changed to <code>-std=gnu17</code>.</li>
> +  </ul>

That seems reasonable.

> Should it say 2017 to avoid confusion, even though it was published as
> 9899:2018?

Maybe something like "2018 edition (<code>__STDC_VERSION__ == 
201710L</code>)".

-- 
Joseph S. Myers
josmyers@redhat.com