I haven't looked at the code at all, I'll leave that up to others. But one thing: While it builds fine in general, I couldn't get the kfileitemaction plugin to build, so that it shows up in dolphin's context menu e.g.
OTOH, the version from git://anongit.kde.org/scratch/thomasfischer/kdiff3.git (kf5 branch) works fine in this regard. Maybe you should add this commit? https://cgit.kde.org/scratch/thomasfischer/kdiff3.git/commit/?h=kf5&id=08f309ef1154232e356515ace4846c932c570940 (untested though, i.e. I'm not sure that this alone will help) Thanks. Kind Regards, Wolfgang