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).

You'll also need to handle this: https://github.com/saghul/pycares/blob/master/src/pycares.h#L18-L20