vtk: fixes for 10.8 and 10.6 with python wrapping

For 10.8 : https://github.com/mxcl/homebrew/issues/16646
For 10.6 : https://github.com/mxcl/homebrew/issues/14871

Closes #180.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>