Struggling with getting rid of deprecations

I’m trying to make our project ready for newer gradle (currently it works with gradle 6.x). Our project is Exult (a modern engine for the DOS game Ultima VII) and our Android port (GitHub - exult/exult: Exult is a project to recreate Ultima 7 for modern operating systems, using the game's original plot, data, and graphics files. exult/android at master · exult/exult · GitHub).

The first deprecation was easy (repositories. jcenter). Then I used gradle 7.6.1 to scan and it failed at

FAILURE: Build failed with an exception.

* Where:
Build file '/home/dom/build/android/app/build.gradle' line: 6

* What went wrong:
A problem occurred evaluating project ':app'.
> Could not get unknown property 'EXULT_SOURCE_DIR' for project ':app' of type org.gradle.api.Project.

Any idea how to fix that?
exult/ at master · exult/exult · GitHub ← offending line
exult/settings.gradle at master · exult/exult · GitHub:app
exult/ at master · exult/exult · GitHub ← (EXULT_SOURCE_DIR)