[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>The default mode has been changed to <code>-std=gnu17</code>.</li>
> +  </ul>

The option didn't exist before GCC 8, I think you need to say something 
about the option being added rather than just referring to a new default 
of an option that wasn't in previous versions.

-- 
Joseph S. Myers
josmyers@redhat.com