Add jdk8 build variant
Add jdk8 build variant

Helps when using versions of third-party libraries that are not yet Java
9 compatible and are bothered by the existence of the module-info.class,
such as Jetty 9.4.6 with its runtime classpath scanning.
JOML-CI/JOML,JOML-CI/JOML,JOML-CI/JOML