Changes against math/vtk6:
* Use DISTVERSION
* Update COMMENT
* Add to USES
* Remove the SHARED option: all libraries should be shared, no need for the option to do otherwise
* Remove the PYTHON option: python binding should be a separate port
* Modernize the port: use helpers instead of .if/.endif
* Use autoplist: too many files with too complex dependencies on options
* Update USE_QT to include missing parts
