The only other C based client I’m aware of is the OpenBSD acme-client but it’s a fairly heavyweight client that has an interesting architecture focused on security perhaps at the cost of simplicity. It’s also not portable as-is. There is a portable version but until recently it was ~abandonware. A new maintained fork of the portable version is here: https://github.com/graywolf/acme-client-portable I believe it is BSD licensed.
If you’re willing to compromise on needing a client written in C to allow one written in C++ then acme-lw might be a better starting point. It looks a lot more straight forward to use as a library and is licensed under MIT.