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

[Michael Scott]

> The goal of the old logic was to avoid populating the settings in the
> cache if they are left as default.  Instead you could check whether any
> "dev" value has been set *after* parsing the options and if not then
> set it to the default.

Ah okay I understand, that's no problem.

I've pretty much finished splitting up the original patch into about 
seven separate patches, should be able to submit later on this week.

Cheers,
Michael