Explicitly enable author warnings by default, via the
cmake::GetSuppressDevWarnings method, which signals suppression
is turned off unless the CMake variables are set as required.
Add test cases for author and deprecated messages displayed by
default.
By default, the message is not issued. If CMAKE_ERROR_DEPRECATED
is on, the message is fatal. If CMAKE_WARN_DEPRECATED is on, the
message is a warning.
Michael Scott
Stephen Kelly