Work around re2 2023-07-01 compilation failures on macOS

What does this merge request do and why?

re2 2023-07-01 directly depends on abseil (https://github.com/Homebrew/homebrew-core/commit/ead94abbffe887e8eaa43c6074c99ab202db0982). To make the re2 gem work now, update the build.re2 bundle config to include:

1. A path to the abseil include headers. The default /opt/homebrew/include path isn't included since --with-re2-dir is used.
2. -std=std++20 in the compiler flags since since abseil needs C++17 or later. macOS defaults to C++14.

Upstream issue: https://github.com/mudge/re2/pull/65

Relates to #1877 (closed)

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.