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

[Michael Scott]

Hi Brad,

> The hunk I included at the bottom of the message
Ah yeah, sorry missed that part in the message, I understand the change 
that needs to be done now.

> The current caching logic still needs some work
One idea that comes to mind, is to change it so that it doesn't enable 
dev/deprecated warnings by default by setting the DiagLevels map, 
instead we create a specific function to get the status of dev warnings 
suppression, which checks the cache variables (now set only if user 
explicitly used -W options) but by default returns a value to signify 
enable dev warnings. I think this would get around the mentioned issue, 
but needs some playing about with to see if its a solution.

Cheers,
Michael