I went ahead and compiled and installed the current version of RTMIDI, and now it fails with something else that looks like some file is not compatible with current CMake:

    CMake Error at cmake/FindRTMIDI.cmake:51 (string):
      string sub-command REGEX, mode REPLACE needs at least 6 arguments total to
      command.

There might be issues even with normal linux boxes, just because of bit rot.