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

[Michael Scott]

Hi Brad,

I was thinking of coming back to issue of the deprecation and author 
message options, now that CMake 3.4 has been released, is now a suitable 
time for it?

I was thinking of ideas and one came to mind, how does the following sound?

We modify cmake::IssueMessage to check the relevant CMake variables to 
determine if the message should be output and at which level, we try to 
get the script provided values if possible but handle the case where we 
can't. We also modify IssueMessage to use 
cmSystemTools::SetFatalErrorOccured (or SetErrorOccured if that's 
preferred) if a warning has been turned into an error. Finally we modify 
the users of IssueMessage, to check the error occured state using 
cmSystemTools, and use that to determine if an error has occured (rather 
than the message level going into IssueMessage) and if the return value 
should be changed accordingly.

Cheers,
Michael