The "build.xml" file in the root directory of the source zip file is an Ant build script. Ant is a de-facto build tool for simple Java projects. You can download and read about Ant here:http://ant.apache.org/
Follow that link and download Ant. Once Ant is installed (and added to your PATH, should also be in the install instructions on the site and/or a README file in the Ant download), from a command prompt, you should be able to build RSyntaxTextArea by just typing "ant" and hitting Enter.
Hope this helps!