cmake: export and install library stag_core

[meyerj]

Thanks for your nice package, or more specifically for porting STag and integrating it with ROS!

Some suggested changes to CMakeLists.txt:

• List library stag_core in catkin_package(LIBRARIES ...), such that other derived packages can link to it. A library named stag_ros is not built nor installed by this package, so trying to

  find_package(catkin REQUIRED COMPONENTS stag_ros)

  as-is would trigger a "library not found" error in catkin. See here for the official documentation.

  Background: We are integrating the STag in another package built with catkin. So starting from stag_ros is convenient, but we would like to link to library stag_core only, and not use the nodelets or node executables as-is.

• All library targets need to be installed for the package to work with an install-space, not only the executables. Unless the libraries would be linked statically, but that is not the default with catkin.

  If not, running stag_node throws an error:

  $ source /path/to/install/setup.bash
  $ rosrun stag_ros stag_node             
  /path/to/install/lib/stag_ros/stag_node: error while loading shared libraries: libstag_core.so: cannot open shared object file: No such file or directory

  It may still work without, as long as the devel-space is around and has not been cleaned, but not for a binary package built by the OSRF buildfarm, for example.

• _Edit (after https://github.com/usrl-uofsc/stag_ros/pull/18/commits/13aa45ee22606a13b8a1d5b87b908146a29966eb):_ Also the header files need to be installed.

If you prefer not to export any library target and/or header file to downstream packages, that is also fine. Then we can fork STag directly and add a package.xml and CMakeLists.txt file there. I would still recommend to leave the catkin_package() call empty then, because there is nothing to export, and for simply running the nodes or nodelets that is not required.

[meyerj]

• It may still work without, as long as the devel-space is around and has not been cleaned, but not for a binary package built by the OSRF buildfarm, for example.

Confirmed that the latest ros-noetic-stag-ros binary package for Ubuntu Focal, version 0.3.9-3, indeed does not work:

johannes@im-laptop-007:~$ source /opt/ros/noetic/setup.bash
johannes@im-laptop-007:~$ rosrun stag_ros stag_node
/opt/ros/noetic/lib/stag_ros/stag_node: error while loading shared libraries: libstag_core.so: cannot open shared object file: No such file or directory
johannes@im-laptop-007:~$ rosrun stag_ros stag_nodelet_runnable 
[ INFO] [1675885893.643372928]: Initializing nodelet with 8 worker threads.
[ERROR] [1675885893.655363470]: Failed to load nodelet [/stag_nodelet] of type [stag_ros/stag_nodelet] even after refreshing the cache: Could not find library corresponding to plugin stag_ros/stag_nodelet. Make sure the plugin description XML file has the correct name of the library and that the library actually exists.
[ERROR] [1675885893.655398627]: The error before refreshing the cache was: Could not find library corresponding to plugin stag_ros/stag_nodelet. Make sure the plugin description XML file has the correct name of the library and that the library actually exists.

[MikeK4y]

Thank you for your contribution!