arrow

ARROW-5659: [C++] Add support for finding OpenSSL installed by Homebrew

Homebrew OpenSSL is keg-only formula. It means that it's not enabled
automatically. We need to enable it manually.

This is needed to build Flight smoothly on macOS.

This also cleans up Homebrew related codes.

Author: Sutou Kouhei <kou@clear-code.com>

Closes #4621 from kou/cpp-openssl-macos and squashes the following commits:

9807491dc <Sutou Kouhei>  Add support for finding OpenSSL installed by Homebrew