[wwwdocs] Document -fcommon default change

[Gerald Pfeifer]

On Tue, 7 Jan 2020, Wilco Dijkstra wrote:
> Thanks for the suggestions, I've reworded it as:

> +  <li>GCC now defaults to <code>-fno-common</code>.  As a result, global
> +      variable accesses are more efficient on various targets.  In C, global
> +      variables with multiple tentative definitions now result in linker errors.
> +      With <code>-fcommon</code> such definitions are silently merged during
> +      linking.
> +  </li>

Thank you, this patchset is okay (also the other parts).


(Ideally reformat for a bit smaller width since right now the patch
format, which is wider by a few columns, induces line wraps in an 80
column view, but that is optional.)

Thank you,
Gerald