systemd-python: use python${PYTHON_VERSION}-config as python-config

This is the usual setup, where pythonX.Y and pythonX.Y-config go
together. Using python-config with python3 will only lead to
confusion.

--libs is changed to --ldflags, since the latter also includes other
required flags like -L<dir>.

The tests for HAVE_PYTHON and HAVE_PYTHON_DEVEL are separated. It is
possible to have python development libraries installed without the
binary (or to want to build python modules without using python during
the build).

A line is added to the output, to show what flags will be used for
python.

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