All the files in the antlr-3.0/runtime/Java/src/org/antlr/runtime directory are needed to create
a full back-end. You must also copy another group of templates such as antlr-3.0/src/org/antlr/codegen/templates/Java/*.stg. And create antlr-3.0/src/org/antlr/codegen/YourTarget.java if you need to override anything in Target.java.

But it is in fact pretty easy to get started:

• In src/org/antlr/codegen/templates/
  • create a directory Ada95
  • copy Java/Java.stg to Ada95/Ada95.stg

• I recommend building the ANTLR tool 'in place'. Do not create a jar or
  compile/copy to a build directory. When you run it with 'java -cp
  path-to-src-dir ...' it will use the original *.stg file, which you'll edit
  a lot - so rebuilding the tool would be quite a PITA.

• Create a directory antlr-3.0/runtime/Ada95. Here you can put anything you
  need (no need to clone Java 1:1).

• Start with a simple lexer like:

  lexer grammar T; options { language = Ada95; } ZERO: '0';

• Look at the generated code and try to figure out which templates in
  Ada95.stg you have to port to get valid Ada95 code. What I did, is to
  comment out the Java code in all templates replacing it with something like
  FIXME([number]). Then you fix the templates until no FIXME remains in the
  output.

• You'll need a basic implementation of a character stream and base
  recognizer/lexer to get the example running. Just implement the methods
  that are actually needed to get the example running w/o errors.

• You'll either get the feeling "Wow, that was easy!" and move on (that
  happened to me) or "Eeek, what a pain!" and let someone else to the work.