[C++] Require c-ares CMake config

[asfimport]

Because gRPC requires c-ares' CMake config.

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #4783

_{Note: This issue was originally created as ARROW-5827. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4783
#4783