Xcode 10+: how to switch to legacy build system?

Question

There seem to be some problems with the new build system in Xcode 10+. How do I switch to the old (legacy) build system?

Answer 1

You can switch back to the previous (legacy) build system from the menu by File > Workspace Settings... (or Project Settings...):

This brings up a dialog where you can change the Build System to Legacy Build System:

Hat tip to this answer. There's also a video here.