COMP: Re-enable VTKv6 on MacOSX.

Since following commit r23350, issue #3741 is resolved, this commit
re-enables VTKv6 on MacOSX.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23352 3bd1e089-480b-0410-8dfb-8563597acbee