Check whether macro RUBY is okay for protecting ruby-specific onigumo extensions
In issue #12386, I removed the ONIG_CASE_MAPPING macro because we thought that there was no need to keep the Ruby version of Onigumo in sync with the original version (see https://bugs.ruby-lang.org/issues/12386#note-1).
However, there's now a plan to integrate the Ruby-side changes into the original Onigumo version (see https://github.com/k-takata/Onigmo/pull/58). The proposal is to surround the Ruby-specific changes by #ifdef RUBY (see https://github.com/k-takata/Onigmo/issues/66).
We should check whether we are okay with #ifdef RUBY, or we would prefer something else.