add support for using the OpenJDK 8 class library

This ensures that all tests pass when Avian is built with an
openjdk=$path option such that $path points to either OpenJDK 7 or 8.

Note that I have not yet tried using the openjdk-src option with
OpenJDK 8.  I'll work on that next.
add support for openjdk-src builds with OpenJDK 8

I've tested this on Linux with both OpenJDK 7 and 8.  Other platforms
will probably require further changes.
Merge pull request #245 from dicej/jdk8

add support for using the OpenJDK 8 class library