Bug#1012215: gradle-debian-helper: unknown option --add-opens breaks OpenJDK 11 packages

[Emmanuel Bourg]

gradle-debian-helper/2.2 already checks if the JDK supports modules before 
adding the --add-opens options, but it checks the default JDK and not the one 
specified by JAVA_HOME, that's why it fails when OpenJDK 8 is used.