The discussion has veered off topic somewhat with the discussion of build systems and their respective qualities.
My pull request is proposing to change build system from Ant to Maven. All existing functionality in the Ant build has been/will be replicated in the Maven build - including the pre-processing step. There are many advantages to using Maven as opposed to Ant. However, this is orthogonal to the discussion I'd like to have about the pre-processing used during the build process.
Focusing on the aim of the current pre-processing step - which is to select the appropriate spec implementation at compile time. The options seem quite complicated to me as a relative novice in the finer points of the language spec.
Taking a step back, could this not be achieved using annotations and a custom annotation processor to emit some code to select the correct implementation of the spec? This seems more Java-like and not a massive change from the existing code.