[C++] Don't use .pc only in CMake paths for Requires.private #28954
Description
Because they can't be found by raw pkg-config usage.
Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou
PRs and other links:
Note: This issue was originally created as ARROW-13273. Please see the migration documentation for further details.