Clone only latest OpenCV tag if possible

[skvark]

Pulling development version and building it is a bit risky. Separate builds with the dev version can be of course provided.

Versioning of these builds?

[skvark]

OpenCV could be included as a submodule, see: https://github.com/python-pillow/pillow-wheels

[skvark]

Builds use now submodules.