If you try to compile java files which are in a directory containing a space 
(for example in program files), you cannot fork the javac task. 
I've worked around the problem by updating the following ant src file:

main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java

Changed the method executeExternalCompile(String[] args, int firstFileName)  to 
put quotes around the java file names.


So the following :

   for (int i = firstFileName; i < args.length; i++) {
      out.println(args[i]);
   }

becomes 

    for (int i = firstFileName; i < args.length; i++) {
      if ( i==0)
      {
         out.println(args[i]);
      }
      else 
      {
        args[i] = args[i].replace('\\', '/');
        out.println( "\"" + args[i] + "\"");
      }
   }