Thanks a lot for explaining the situation.  From a distance it *appears* as if opm-parser's dependency relation with cJSON is similar to opm-autodiff's dependency relation with Eigen.  I'm afraid I don't know how to best express that relation in a CMake rule so I'll defer to someone else (@rolk? @andlaus? @akva2?) to give technical advice.

At least I now know what problem is being solved here.