Error compiling LLBuild on macos Mojave

I am attempting to build swift using the instructions here:

And I am getting an error while building llbuild-macosx-x86_64:

swift-source/llbuild/lib/Basic/Subprocess.cpp:92:14: error: no member named 'posix_spawn_file_actions_addchdir_np' in the global namespace; did you mean 'posix_spawn_file_actions_addchdir'?
return ::posix_spawn_file_actions_addchdir_np(file_actions, path);
       ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         posix_spawn_file_actions_addchdir
/swift-source/llbuild/lib/Basic/Subprocess.cpp:85:12: note: 'posix_spawn_file_actions_addchdir' declared here
static int posix_spawn_file_actions_addchdir(posix_spawn_file_actions_t * __restrict file_actions,
       ^

How do I mitigate this error? Am I missing an environment variable or something?

By the way, I'm building the current contents of the master branch, on macOS 10.14.5

This should be fixed by the following change: Guard new posix_spawn_* APIs with the macOS 10.15 SDK macro · apple/swift-llbuild@9cf3059 · GitHub

Thanks, that did the trick!