I ended up pulling the stellarsolver repo from github.
After compiling the sources I installed it over the existing installation which was already stellarsolver version 1.6.
But the CMakeLists.txt had 4 changes. This seems to be crucial.
My local project is up to date and now also in a working state.
FWIW, I did try and update the KStars README file to address this stellarsolver cmake issue.
E.g. search for stellarsolver in the bottom/README section on invent.kde.org/education/kstars
If you feel there are better ways to phrase things, please let me know and I'll update the instructions.
Setting up another Kstars client I ran into it again. No problem, just noticed that the issue should be mentioned in the Kstars INSTALL file that comes with the git download, too. AFAICS the current version is from Kstars 3.3.7 and does not contain any info regarding Stellarsolver.