It was late, and I was tired. I misunderstood this as a directive to the compiler to do some transformations as the engine was being compiled.
In which case, those who want modules would need to compile with java 9 or better anyway… So, gate the gradle directive on the compiler you are using.
Since this actually just dumps a map/dictionary in the manifest, as @pspeed pointed out, no need to gate. My original comment is therefore a moot point.