I'm curious about the ordering here.  Doesn't this imply that we're looking for opm-parser *before* cJSON?  Wouldn't it be more natural to look for cJSON prior to opm-parser as cJSON is a prerequisite for opm-parser?

I also wonder if the cJSON might not be better placed in `opm-parser-prereqs.cmake` and not in `opm-core-prereqs.cmake`.  Do you intend to make opm-core *directly* dependent upon cJSON?

@rolk: What do you think?