link moveit_robot_model from moveit_test_utils

[v4hn]

The library internally creates RobotModel objects
and thus should link the constructor.

---

I just tried to build a fresh current master workspace on Lunar-Linux with clang++ and hit this error:

Errors     << moveit_ros_planning:make ros/melodic/moveit/logs/moveit_ros_planning/build.make.001.log
/usr/bin/ld: ros/melodic/moveit/install/lib/libmoveit_test_utils.so: undefined reference to `moveit::core::RobotModel::RobotModel(std::shared_ptr<urdf::ModelInterface> const&, std::shared_ptr<srdf::Model const> const&)'
/usr/bin/ld: ros/melodic/moveit/install/lib/libmoveit_test_utils.so: undefined reference to `moveit::core::RobotModel::~RobotModel()'
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [planning_scene_monitor/CMakeFiles/demo_scene.dir/build.make:248: ros/melodic/moveit/devel/.private/moveit_ros_planning/lib/moveit_ros_planning/demo_scene] Error 1
make[1]: *** [CMakeFiles/Makefile2:3183: planning_scene_monitor/CMakeFiles/demo_scene.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:141: all] Error 2

(1) This request simply links libmoveit_robot_model from libmoveit_test_utils.so. Both reside in the same ROS package, so the library can be referenced by literal name here. I seem to recall a debate on whether this link is necessary or not but I can't find it anymore.

(2) It does not really make any sense that a binary like moveit_ros_planning/demo_scene links the tests lib to begin with. But it is probably pulled in as a general library dependency of moveit_core. The concrete so-link might be related to me not using a Debian-based linux distribution.

[rhaschke]

Seems to be reasonable. I don't understand, though, that Travis' clang build doesn't complain.

[v4hn]

Some years ago I encountered comparable issues with non-Debian systems.
Back then this involved gcc and related to a change of the default --as-needed.
At the moment I can't make the problem disappear on my end with the flag, but I guess it might be related to differences between different Linux distributions.