Configure Java compiler class path correctly

- This wasn't being done properly before. The compiler class needed to
  be configured via the -cp option. We use the new "boot.class.path"
  system property and pass that as the -cp option to javac.

Example usage:

1. In a new directory create a "src" subdirectory.

2. Create a "src/Main.java" file with the following contents:

    import clojure.java.api.Clojure;
    import clojure.lang.IFn;
    public class Main {
      public static void main(String [] args) {
        IFn cloj = Clojure.var("clojure.core", "clojure-version");
        System.out.printf("Using Clojure version %s.\n", cloj.invoke());
      }
    }

3. Then build an uberjar:

    $ boot -d org.clojure/clojure:1.6.0 -s src javac uber jar -m Main

4. You can then run the program:

    $ java -jar target/project.jar

5. You should see something like "Using Clojure version 1.6.0." printed.