The "empty" rules are provided for cases in which you have to supply
some value but don't have anything to build.  They are available for
both Groovy-based and make-based builds.

There are two typical reasons why you might use these rules:

 * You wish to have accessed to globally available features such as
   qtest support.  If you have a build item that has qtest-based test
   suites but doesn't actually have to build anything, you can use the
   "empty" rules.

 * You decide which rules to use based on some kind of conditional
   logic.  For example, for a Windows-only build item, you might use
   RULES=ccxx Windows and RULES=empty everywhere else.