- Reduce unnecessary linking

- Reduce unnecessary linking

- Link to libpython only when needed, fixes #242

When pyunbound is enabled while pythonmodule is not (i.e., ./configure
--without-pythonmodule --with-pyunbound), only the Python library
_unbound.so uses Python functions, and main programs (unbound,
unbound-anchor, ...) and libunbound.so do not. This patch removes
unneeded linking.

- Link the Python library _unbound.so to Python only. _unbound.so does
  not directly use libraries used by libunbound. This patch removes
  unneeded linking mentioned in [1]

[1] https://github.com/NLnetLabs/unbound/pull/511#issuecomment-886072003