JDK 9 contains a lot of breaking changes which make our build fail. Investigation with early access builds and release builds uncovered at least the following problems:
GNU Classpath doesn't build out of the box with Java 9 because it still uses source version 1.5 for configure checks
(not verified) GNU Classpath uses _ as an identifier in some places which needs to be changed for the future
-X:bootclasspath was removed which causes gen-interface to fail
the boot image writer needs to be changed to account for the module systems, build VMs crashed with early access builds
It is probably a bad idea to try to patch GNU Classpath at this point. We should focus on getting support for OpenJDK and Java 7 before we try adding support for building with JDK 9.