For policy-specific tests, use the version before the policy was
introduced. Otherwise, use 3.5 where possible.
Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.
Add a new property flag for a target which contains a message regarding
deprecation status.
Add a warning at "Generate" time if a linked target is marked as
deprecated.
Expand ExportImport test to ensure that new property is being set and
passed correctly. Ensure that the message is shown during the
"Generate" step run of the ExportImport test.
