Generate cmake target that links all the libraries to support using target_link_libraries

[tylerjw]

The generated libraries have really long names like control_msgs__rosidl_typesupport_c and there are many for each messages project. In order to support linking against a messages package with target_link_libraries like this:

target_link_libraries(hardware_interface PUBLIC control_msgs::control_msgs)

Could we add a generated cmake library target that links with all of the generated message libraries?

[sloretz]

There is a variable - ${control_msgs_TARGETS} - which contains all targets from all the generators (assuming they export them). That's what I usually use.

There is a top level target already, but right now it's just there to have a way to wait until after message generation completes. Maybe it could be remade to a target that gets exported?