Make sure the following two jars are in your CLASSPATH
antlr-3.0ea8/lib/stringtemplate-2.2b6.jar antlr-3.0ea8/lib/antlr-2.7.6.jar
then jump into antlr-3.0ea8/src directory and then type:
$ javac -d . org/antlr/Tool.java org/antlr/*/*.java org/antlr/*/*/*.java
Takes 9 seconds on my 1Ghz laptop or 4 seconds with jikes. Later I'll have a real build mechanism, though I must admit the one-liner appeals to me. I use Intellij so I never type anything actually to build.