Shoot, actually this doesn’t quite work 
Header paths aren’t passed along as to the executable via addObject().
It was as simple as adding a call to installHeadersDirectory(). See @kibels answer + this for the “complete” answer on how to do this!
Here’s a quick repo I made to demonstrate:
I’m just going to keep populating this repo with various examples of adding “sub-projects” in Zig since docs are light + post to showcase eventually 