setup: add flag to build_ext to control building zstd
Downstream packagers will inevitably want to disable building the
vendored python-zstandard Python package. Rather than force them
to patch setup.py, let's give them a knob to use.
distutils Command classes support defining custom options. It requires
setting certain class attributes (yes, class attributes: instance
attributes don't work because the class type is consulted before it
is instantiated).
We already have a custom child class of build_ext, so we set these
class attributes, implement some scaffolding, and override
build_extensions to filter the Extension instance for the zstd
extension if the `--no-zstd` argument is specified.
Example usage:
$ python setup.py build_ext --no-zstd
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true