Python wrapping is slow on Windows

[ben.boeckel (Ben Boeckel (Kitware))]

@dgobbi

If the hierarchy file timestamp is updated, it triggers everything that consumes the hierarchy file, which causes the wrappers to be rebuilt entirely. We had a separate timestamp file previously (up until VTK 8, I think?) but it was removed to simplify the rules. I’m not eager to add the timestamp file back into the mix.

Maybe only updating the generated source file if the content changes is better? Using the hierarchy files would then be done only as needed, but actually generating the source is generally pretty cheap (compared to compilation).