Set --enable-static for building charlock_holmes

What does this merge request do and why?

charlock_holmes has now upstreamed the changes in static_holmes (https://github.com/brianmario/charlock_holmes/pull/164). In preparation for switching back, set the build flags to link libicu statically for macOS.

How to set up and validate locally

Nothing really to test at this point. You can test that the latest charlock_holmes can do this:

gem install charlock_holmes -- --enable-static

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.