USD runtime loading, GlobalDefinitions error

Having trouble loading USD files at runtime.

According to UE docs:
Universal Scene Description in Unreal Engine | Unreal Engine Documentation

I need to add this line to project.Target.cs:

If I do this, I get the error:
Packaging (Mac): ERROR: project modifies the value of GlobalDefinitions. This is not allowed, as project has build products in common with UE4Game.

Elsewhere on the web it is suggested to also add:
BuildEnvironment = TargetBuildEnvironment.Unique;

However this results in error:
ERROR: Targets with a unique build environment cannot be built an installed engine.

Anybody have clues as to what is wrong and how to solve?