Unable to build tool for Processing 4.0.1

I am trying to build GUI Builder for Processing 4.0.1 using Eclipse.

With PS v3 this has never been a problem but now the pde is compiled to Java 11 (major version 55) so I get

import processing.app.Base;
    [javac]                      ^
    [javac]   bad class file: /Users/peter/Documents/e-workspaces/quarks ws/lib4.0.1/pde.jar(processing/app/Base.class)
    [javac]     class file has wrong version 55.0, should be 52.0
    [javac]     Please remove or make sure it appears in the correct subdirectory of the classpath.

I have changed the project’s Java compliance to 11 but no matter what I do I can’t get past this point in the Ant build.