[C++] MinGW builds failing due to CARES-related toolchain issue

[asfimport]

This just started occurring today I think

Example run https://github.com/apache/arrow/pull/6774/checks?check_run_id=547420903

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
CARES_INCLUDE_DIR

Reporter: Wes McKinney / @wesm
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #6778

_{Note: This issue was originally created as ARROW-8280. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 6778
#6778