Java version and JRuby port

[Bruce Albrecht]

When the JRuby port is installed, it updates the jruby script to force 
the JAVA_HOME to be the JAVA_HOME the port was built with.  I've got 
both diablo-jdk1.6.0 and openjdk6 installed, but I can't figure out how 
to force the JRuby port to use openjdk6, and I can't override the 
JAVA_HOME unless I'm willing to modify the jruby script. Why is this 
port set up this way?  What's the rationale for not allowing JAVA_HOME 
to be overridden?