This would reflect the dependency on opengtl-package, and not only the build-option "OpenCTL". AFAIR in the early days, there were two packages: openctl and opengtl. It looks like they got merged into opengtl (at least opengtl also installs openctl library+headers) and krita only used the openctl-part.
Done in 2.4.91 and later, thanks. BTW, if you would like to get more involved, just tell me and head over to https://recruiting.gentoo.org/ to make yourself an account there and start abusing the questions... :)