Add builders and construction variables for the DistTar builder.

def generate(env):
    """
    Add builders and construction variables for the DistTar builder.
    """

    disttar_action=SCons.Action.Action(disttar, disttar_string)
    env['BUILDERS']['DistTar'] =  Builder(
            action=disttar_action
            , emitter=disttar_emitter
            , suffix = disttar_suffix
            , target_factory = env.fs.Entry
    )

    env.AppendUnique(
        DISTTAR_FORMAT = 'gz'
    )