I didn't get the "it doesn't count". It seems like Jenkins is checking for a dependency which is openjdk related, and I don't see how I can ignore it. It should maybe be part of the package?
Jenkins doesn't check for the actual presence of Java 8. It declares a dependency on a generic package that any Java 8 should provide. Unfortunately whatever you installed doesn't provide it. E.g. for our RPM packages, we've given up on declaring a dependency and just document the following:
You will need to explicitly install a Java runtime environment, because Oracle's Java RPMs are incorrect and fail to register as providing a java dependency. Thus, adding an explicit dependency requirement on Java would force installation of the OpenJDK JVM.
In the case of Debian, Ubuntu (well, 16.04), etc. there are official Java 8 packages, and they satisfy the Jenkins-declared dependency.
Your package manager should allow you to ignore unsatisfied dependencies, so that may be a feasible way forward.