There is another way of doing this with the scala library `sys.props.get("akka.genjavadoc.enabled").getOrElse("false").toBoolean` Maybe we should use that throughout the build.

SBT used to support system properties `system[Boolean]("java.awt.headless")` but it was [quite some time ago](http://www.scala-sbt.org/0.7.7/docs/Properties.html).