check and link against libatomic

[christianrauch]

Link against libatomic. Fixes #171 .

[christianrauch]

@clalancette @sloretz Can you have a look at this?
I would like to propose the same linking fix for a couple of other packages (and the dashing branch) where I encountered the same libatomic linking issue.

[sloretz]

Thanks for the PR. Downstream packages will need atomic too. I think this can be done automatically by adding a call to ament_export_library_names() here.

if(HAVE_LIBATOMICS)
  ament_export_library_names(atomic)
endif()

[christianrauch]

I got an error with ament_export_library_names(atomic), but ament_export_libraries(atomic) did work.

[sloretz]

CI (building all, testing just rcutils)

• Linux
• Linux-aarch64
• macOS
• Windows

[sloretz]

Thanks for the PR @christianrauch !

[dirk-thomas]

Every package using rcutils reports the following CMake warning for me locally:

CMake Warning at <ws>/install/rcutils/share/rcutils/cmake/ament_cmake_export_libraries-extras.cmake:116 (message):
  Package 'rcutils' exports library 'atomic' which couldn't be found
Call Stack (most recent call first):
  <ws>/install/rcutils/share/rcutils/cmake/rcutilsConfig.cmake:38 (include)
  CMakeLists.txt:34 (find_package)

[sloretz]

Every package using rcutils reports the following CMake warning for me locally:

Looks like I forgot to include a repos URL in CI. Re-running to check if it affects the build farm too

EDIT:
I see the cmake warning in the nightly, reverting

https://ci.ros2.org/view/nightly/job/nightly_linux_debug/1298/consoleText