Request for population of INTERFACE_INCLUDE_DIRECTORIES in export

Hello all,

If you work on both projects, you'll find this very similar to the ITK request (https://issues.itk.org/jira/browse/ITK-3470).

This is a request for VTK to populate the INTERFACE_INCLUDE_DIRECTORIES property on targets so that linking to an VTK target correctly adds all includes.

I'm sure you already know, but just in case, this is accomplished via target_include_directories(<Target> PUBLIC|PRIVATE|INTERFACE <Directories>)