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

[Michael Scott]

I've created a fix for the issue of -Wno-dev and -Wno-deprecated options 
not being honoured, and extended the tests to cover this additional 
scenario.

However I'm having an issue with determining if variables are set in 
cmake.cxx. The initial fix checked for the variables using the 
"GetCacheEntryValue" method in cmState, but I found that this didn't 
work when the variable (say CMAKE_WARN_DEPRECATED) was set in a .cmake 
file, as is the case for the "RunCMake/message/warnmessage.cmake". I 
toyed around with trying to get a hold of a cmMakefile instance and 
check the state of the variable via that, by getting the first object of 
the global generator makefiles, but this didn't always seem to work. 
Would you be able to advise on what the correct way is to get the 
current value of a variable, in the cmake class? I feel like I'm missing 
something simple here, as I'm still not too familiar with the code.

Cheers,
Michael