Make the message suppression more consistent, by adding a check
for the message related CMake variables in cmake::IssueMessage,
which allows callers of IssueMessage other than the message
command to behave as expected. Also added a check for
CMAKE_SUPPRESS_DEVELOPER_WARNINGS in the message command to
mirror the deprecated message type behaviour.
Added a 'force' flag to the cmake::IssueMessage method, to
make the message suppression consistent, when setting the
message related CMake variables directly in a CMake file.
Expand message command tests to cover the AUTHOR_WARNING message
type as well.
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