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).

Ah, I see. We probably want to import the bundled one when making a static build, which will be the only option on Windows, so that's ok.