fixes for slicer4.2

Yo Daniel!

Here are the changes - note that [1] is only a transient issue that is fixed if you use the 'just right' version of cmake - I understand that the current nightly build of cmake works with the latest Xcode and doesn't need this hack, but it was required when I was doing the testing.  We'll want to back it out eventually though.

Also, I never did figure out the extension packaging issues due to the cmake issues I mentioned -- I think packaging will work on linux and (maybe?) windows, but haven't tried yet.  Jc and I spent a fair amount of today's hangout talking over the various issues, so it's been a good exercise.

-Steve

[1] https://github.com/pieper/VMTK-in-3D-Slicer/commit/a54576a4fa3632799b75993cc3fe54495677c939