Upgrading the JDK 9-based build to use JDK 9.0.4 has revealed that
Gradle 4.0.x doesn't work with it as it fails to parse the 9.0.4
version number.
This commit disables building the Gradle plugin on Java 9 (as we
had already done for Java 10) until we decide what to do
(see gh-12333).
2bcf29913d