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

[Michael Scott]

> The -Wdev/-Wno-dev options have always been for all messages
> output by any part of CMake.  This needs to be preserved/corrected.
Okay, I've fixed the change to cmake::PrintMessagePreamble, so that 
it'll stop warning or error dev or deprecated messages getting printed 
out when the relevant CMake variables have been set. I just need to add 
some more test cases to cover this and I'll submit a patch.

> Likely yes.  This may require sweeping changes to address.  We're
> too close to the freeze for 3.4 to do this now.  Perhaps we should
> simply remove -Werr-dev for now.

Yes the -Werr-dev, -Wno-err-dev, -Werr-deprectated and 
-Wno-err-deprecated may be trickier than expected to get behaving as 
intended. I'll try and get a better idea of the users of IssueMessage 
and see if some ideas come to mind. Removing them would be fair enough 
at this point, better to not include them than to include broken 
functionality I'm sure. Do new features, such as -Werr-dev, have to be 
sent by October 1st is it?

Cheers,
Michael