It seems ENABLE_LIBHEIF is not mentioned anywhere in the source code or build files, so the proper fix would be to remove it. It may have been retained to avoid breaking package maintainers' scripts.

HEIF support is now provided by a qt imageformat plugin such as kimageformats.

I remember @novomesk mentioned that`` ENABLE_HEIF is used only for MSVC builds.

My mistake, I searched for the wrong string. Yes, it is in the msvc cmake files. So it can be omitted from unix build target so it won't show when it doesn't have any effect.

IF(libheif_FOUND) will be removed. It is most probably some old remnant from the times, when nomacs built qt-heif-image-plugin

On Linux we recommend to use heif plugin from kimageformats package.