[C++] Findre2Alt.cmake did not honor RE2_ROOT

[asfimport]

In Findre2Alt.cmake, cmake module find re2 package config first, which is likely to find system re2 package first over my customized re2 library. We should check RE2_ROOT variable first, and if there is no value existed, then we should search for system re2.

Reporter: Haowei Yu / @sfc-gh-hyu
Assignee: Haowei Yu / @sfc-gh-hyu

PRs and other links:

• GitHub Pull Request #12513

_{Note: This issue was originally created as ARROW-15782. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 12513
#12513