Add scalac as a build dependency

It wasn't listed before and pbuilder/cowbuilder would end up rightfully
complaining

Change-Id: I90244896874461bc498a508caa4bc69c665ab464
Check KAFKA_START before JAVA_HOME

Currently, if Java hasn't yet been set up when this package is installed,
installation will fail when the init script checks for JAVA_HOME.

Instead, check for KAFKA_START first---it will be 'no' immediately upon
installation---and abort cleanly before failing to find Java.
Depend on any java7 jre/jdk