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

[Michael Scott]

Looking at the cmMessageCommand::InitialPass and 
cmake::PrintMessagePreamble code, if we want to mirror the deprecation 
message behaviour, I'm tempted to suggest we also modify the message 
mode "AUTHOR_WARNING" to be "AUTHOR" instead. It would make the mode 
clearer on it's new behaviour and allow the code to be more consistent 
with regards to dev and deprecated.

I imagine this might be a big user affecting change though, so it might 
be better to not do that and just make AUTHOR_WARNING cause fatal error 
messages depending on the state of the associated cmake variables. What 
are your thoughts on this?

P.S. Sorry my previous email broke the message threading, I'm replying 
in a different way this time but please let me know if I'm still 
breaking the message threading.

Cheers,
Michael