I ran into this as well, and had some more luck following this stack overflow post.
The tl;dr is that XCode is not placing key header files in /usr/include and instead bundling them with the MacOS SDK. They provide a program that will move them there for you /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg.
You can read about it a bit more in their release notes.
Hope this helps someone else!