[PATCH, wwwdocs] Add gcc-7/porting_to.html, Fortran changes

[Gerald Pfeifer]

On Tue, 3 Jan 2017, Janne Blomqvist wrote:
> the attached patch mentions some changes in the Fortran frontend for 
> the GCC 7 cycle, and also adds the so-far missing gcc-7/porting_to.html 
> file.

Thank you, Janne.

> Ok to commit?

Yes, modulo Joseph's comment and a missing </p> towards the end
of porting_to.html.

(I am not sure <pre>...</pre> can sit inside a <p>...</p> environment,
and so you may need to alternate the two instead of nesting them, though
if it validates fine, that's okay of course.)

Gerald