jdk16 javadoc tool now usable on Derby

[Rick Hillegas]

I have checked in a change to the way that we generate javadoc: If 
ant.properties points at a jdk16 installation, then we use the jdk16 
javadoc tool and include JDBC4 support classes in derbydocs and in the 
published api. If ant.properties does not point at a jdk16 installation, 
then we use the javadoc tool in $JAVA_HOME and we exclude the JDBC4 
support classes.

Please let me know if you have problems generating javadoc.
Thanks,
-Rick