Adds cmake build system support to C-Ares.

The patch does not modify any source files, it only adds 3 new files
(CMakelists.txt, ares_build.h.cmake, ares_config.h.cmake) which form the
build system.  I've tried to go through as much of the autotools tests and
extracted what I thought was appropriate, though many of the tests aren't
as in-depth in CMake as they are for autotools ... it is unclear why some
of them exist at all, I'm guessing for legacy systems that CMake probably
doesn't support anyhow.

Building the library, and examples (adig, ahost, acountry) plus installation
should work across a large number of tested platforms.  The tests have not
yet been integrated.