A fix for Proguard not working on Mac OS X with Oracle Java.

This is an alternate fix to commit ec49808. I didn't realize when I
did this that he submitted his fix a couple of days earlier.  Please
consider this one, because it checks and manipulates file paths using
only the java.io.File class, rather than by concatenating strings.
Also, if a particular JVM library doesn't exist, it isn't included.