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?
(untested though, i.e. I'm not sure that this alone will help)