Apple platforms are moving to "workspace" builds which build the entire WebKit stack using a single xcodebuild invocation. In build-webkit, this behavior is controlled by the `--use-workspace` argument, which was added in r<> and will become the default. AFAIK, there's not a good reason to continue to support sequential builds after the migration is complete. We can remove the sequential build logic in build-webkit once the dust has settled
--use-workspace was added r288632.
<rdar://problem/94852818>