[vtkusers] Simultaneously build Python 2&3 interface

[Ben Boeckel]

On Sun, Oct 04, 2015 at 19:51:56 +0200, Matthias Blaicher wrote:
> I'd like to start saying that I'm really glad that there is work on a
> Python 3 binding. VTK is the last showstopper for me to convert to
> Python 3 and I can hardly wait until it's in an official release (6.4?).
> 
> I'm unclear though, how I'm supposed to build a python 2 and python 3
> interface at the same time. Is this even possible right now? Hints on
> how to do this would be greatly appreciated!

Right now, use two build trees. A single build tree supporting both is
not feasible currently. Adding suffixes to Python-dependent modules such
as vtkFiltersPython will need to be done so they don't conflict with
each other, but other than that, getting a multi-python-version build
would require major changes to the module system (and FindPython too)
for everything to work properly.

--Ben