Add message dependencies only if in source

[stonier]

Getting

CMake Warning (dev) at /opt/groot/rosjava/devel/share/genjava/cmake/genjava-catkin-api.cmake:51 (add_dependencies):
  Policy CMP0046 is not set: Error on non-existent dependency in
  add_dependencies.  Run "cmake --help-policy CMP0046" for policy details.
  Use the cmake_policy command to set the policy and suppress this warning.

  The dependency target "stereo_msgs_generate_messages" of target
  "rosjava_messages_generate_artifacts" does not exist.
Call Stack (most recent call first):
  rosjava_messages/CMakeLists.txt:14 (generate_rosjava_messages)
This warning is for project developers.  Use -Wno-dev to suppress it.

when the underlying stereo_msgs is installed as a deb, but not in the source workspace.