Remove dependency to parser when using JavaCC

[Niklas Fors]

When using JavaCC with the option --jjtree, JastAdd also requires the option --grammar, which is the name of the generated parser. The latter is used when generating JavaCC visitor methods, which is tied to the specified parser. This is problematic when one abstract grammar is used together with several parsers, because it results in a dependency to only one of the parsers.

This issue is about making the dependency to the parser optional, allowing several parsers to be used together with one abstract grammar.

[Niklas Fors]

This was fixed in 1a52582, making the option --grammar optional. This is a backward-compatible enhancement.