Export threading library via extras and not ament_export_libraries to avoid warnings when cross-compiling

[esteve]

This PR is based on the work that @anup-pem did internally at Apex. He found that cross compiling ROS2 caused the warning below, which doesn't show up when compiling natively because pthread is found on the host but not on the sysroot.

The fix just removes the ament_export_libraries(pthread) call and delegates the logic to the extras file.

/cc @filiperinaldi @lmayencourt

--- stderr: velodyne_driver                                  
CMake Warning at /home/anup.pemmaiah/2394/apex_ws/install/share/rmw_implementation/cmake/ament_cmake_export_libraries-extras.cmake:116 (message):
  Package 'rmw_implementation' exports library 'pthread' which couldn't be
  found
:
:
---

[wjwwood]

Makes sense to me, based on the idea that ament_export_libraries should just be used with libraries you create yourself (which may or may not be correct).

I think it would be good to get a 👍 from @dirk-thomas too.

[wjwwood]

CI:

• Linux
• Linux-aarch64
• macOS
• Windows

[wjwwood]

The test failures seem to be unrelated. The test_cli one's I'm not sure about, but the sros2 one is one that I see on our nightly repeated jobs.

[esteve]

@dirk-thomas I've addressed your feedback, are you ok with the changes?

[esteve]

Thanks @wjwwood and @dirk-thomas for the review.

[dirk-thomas]

Thank you for the patch.