meta-darwin requires that we extract some SDK files from a Mac OS system with the Xcode command-line development tools installed. It would be very useful to be able to remove this requirement, not least of all because we can't make meta-darwin builds on the project's public autobuilder as is due to the fact that we can't distribute the Mac OS runtime files.
I'm afraid this is not possible, or at least it is not trivial. There are dozens of header files that are needed. As we cannot distribute the extracted files, the onus of extraction is put up the user. meta-darwin layer README contains the instructions on how to do this. However, it is possible there is an alternative out there somewhere in the open source community (similar to mingw, which provide their own set of header files in order not to infringe on MS copyright). I am not aware of anything like that, though.
No one is known to be using or improving meta-darwin so we'll close this bug and likely remove the meta-darwin layer.