configure: remove shared cares build option

Bundled cares differs from upstream which will result
in a compilation error when trying to used a shared cares.

Fixes: https://github.com/joyent/node/pull/8786
PR-URL: https://github.com/iojs/io.js/pull/38
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>