In Ant buildfile, use properties instead of hardcoded value.
When a property is available in the build file, that property should be used instead of a hardcoded value. This allows one to do overrides without changing the value.
Openfire 4.0.0 introduced a change to the parent plugin build process. There, a parent plugin is compiled using Java 1.5 (hardcoded), instead of using the available property values.