build with system libcares and use pkg_config

I added support for building against the system libcares and tested this configuration against python 2.7.9 and 3.4.1 .
The static bundled libcares is activated via the setuptool option ````ext-static```` (untested build, only the logic is tested).

I was reading https://python-packaging-user-guide.readthedocs.org/en/latest/index.html and https://www.pypa.io/en/latest/peps/#pep453s or https://www.python.org/dev/peps/pep-0453/ says that setuptools is available at least in python 2.7.9.
Will put it in an try block