cmake find_gradle doesn't work on macosx

This is just simple cmake code, so possibilities for problems are pretty small. The only thing I can think of right now is that `gradlew` is simply nonexistant at the root of a `catkin_create_rosjava_package`'d package.

This would imply the code used to generate gradlew may not be working.

`Louise` can you confirm that there is no gradlew there after you `catkin_create_rosjava_package`?
