heads up: build converted to JDK 9

[Rick Hillegas]

My last checkin (derby-6974-01-ab-deprecateJDK8.diff, svn 1813709) 
converted the Derby build so that Derby can only be compiled into Java 9 
byte code. The trunk no longer runs on older JVMs.

Automated continuous integration rigs will need to disable test cycles 
involving the development trunk and pre-JDK9 platforms.

Please let me know if you encounter any problems.

Thanks,

-Rick