Fix build-time Python configuration

Fix build-time Python configuration

Apply various fixes and tweaks to Python configuration logic implemented
in the "configure" script:

  - Prevent PYTHON_INSTALL_DIR, which holds the value passed to the
    --with-python-install-dir option, from being set to "unspec" by
    default as this breaks installing Python modules when the
    --with-python-install-dir option is not used.

  - Make the --with-python-install-dir option also work when the Python
    interpreter is specified explicitly (using --with-python=<...>).

  - Remove dnspython dependency which was erroneously introduced in
    commit 31b0dc1f204d8f7520145f21e8ea46d1466412a7: no installed Python
    module depends on dnspython, it is only used in system tests, for
    which dedicated scripts exist that check whether dnspython is
    available and act accordingly.

  - Improve contents and placement of error messages.

  - Reduce duplication of code checking Python dependencies.

  - Use Autoconf macros AS_CASE() and AS_IF() instead of plain shell
    code.

  - Update comments.  Capitalize the word "Python" when referring to the
    language itself rather than a specific executable.