Fixes for bild.py

The Maven build is set up with "sane defaults" throughout. The command to build everything is:

    mvn clean install

If you are specifically looking to work with the *Python* runtime, I have a fork of the library at https://github.com/sharwell/antlr4-python2 and https://github.com/sharwell/antlr4-python3 which is configured to use a similar Maven build with defaults.

If you are looking to work on any other target which is open source, let me know and I'll be glad to prepare a **pom.xml** for you which requires no modification (simply open it your IDE of choice and everything works).