Further tweaks:

  - Remove dnspython dependency which was erroneously introduced in
    commit 31b0dc1f: no installed Python
    module depends on dnspython, it is only used in system tests, for
    which there are separate scripts which check whether dnspython is
    available and act accordingly.

  - Print a "checking for Python support... disabled" message when
    --without-python is used.

  - Add "2>/dev/null" to the checks for Python dependencies to prevent
    console output from being mangled when the user supplies a bogus
    absolute path to --with-python.

  - Update the top comment so that it matches post-!980 reality.

  - Add a comment to explain "unset ac_cv_path_PYTHON".

  - s/python/Python/; when referring to the language itself rather than a
    specific executable.

  - Reorder test{argparse,minvers,ply} variables (purely cosmetic).