Use of ANTLR grammar instead of json-formatted grammar

Hi,

Is there a way to produce the generator and mutator from an ANTLR grammar instead of a json-formatted grammar? I know that the antlr grammar is generated and used in intermediate steps, so there should be a way to initiate the process directly with ANTLR, example:

```
make GRAMMAR_FILE=grammars/ruby.g4
```

Thank you.