I've recently upgraded to Ubuntu 20.04 and I see that now `libcmark` is packaged separately, unfortunately it has a broken pkg-config file. I was also excited to see that there's a Github fork `libcmark-gfm` that features Github Flavoured Markdown, which would add support for some of the widely requested features (ex. tables), but again, unfortunately the pkg-config file for this one is entirely missing (it's upstream but not in the Ubuntu package).
Grrrrr....