Hello Timo, Thanks for the bug report.
The builds succeeeds if I use the flag -DCMAKE_POLICY_VERSION_MINIMUM=3.5 in dh_auto_configure or if I patch the cmake scripts files to require CMake minimum 3.10.
I've opened a bug upstream [1] to get their feedback on this issue and I will apply the change accordingly.
/Nicolas [1] https://github.com/json-c/json-c/issues/896
