I feel strongly that you shouldn't go looking for external code that does simple things like this. You can reimplement the relevant parts of mkdir -p in 20 lines of C code, and I think that's what you should do if the standard library doesn't have it.

Having the code inline makes it easy to audit, and prevents problems when a dependency changes 5 years later.

It also trains people to write proper C code. Writing loops and checking errno should be second nature for a C programmer, and doing it often helps you learn.