[cmake-developers] Add command line options for deprecation message control

[Michael Scott]

> In particular,
> I moved the hunk below forward to "Modify dev warning options to affect
> deprecated warnings", the commit where it actually becomes needed
> because the default changes.
I'm not sure I follow, which hunk are we talking about here?

> Please add a Help/release/dev/cmake-W-options.rst file with release
> notes for these changes.
Okay no problem.

> This
> means that adding "-Wno-dev" on a non-initial configuration will not
> disable deprecation warnings because CMAKE_WARN_DEPRECATED is already
> cached even though no explicit -W option was given before.
Ah that's a good point, I hadn't considered that set of steps, thanks.

> As a user
> I may expect those options to be loaded from the current cache in each
> build tree I select.
Okay well I'll remove the related patch from the set for now and come 
back to it after we've got the main sorted out and applied.

Cheers,
Michael