Regarding this, we had some discussion with @linuxmaniac about the paths in general. CMake offers https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html that has the standard definitions for the usual paths.
Therefore I am not really sure if we need to introduce new variables like `prefix` and `run_dir`. User can supply the `LOCALSTATEDIR` or `RUNSTATEDIR` via `-Dvar='path'` and then figure the best way to provide that to kamailio.