[CMake] target_link_libraries - private public interface rules

[Brad King]

On 08/22/2017 11:36 AM, Robert Maynard wrote:
> The `Transitive Usage Requirements` section of the cmake-buildsystem
> documentation quickly covers this idea but it could be elaborated on.
> This might be a good place for a concrete example.

Yes.  Showing the transitive header case as an example would be good
to show when a dependency should be PUBLIC instead of PRIVATE.

-Brad