Add stdc++fs as a target link library for clang compiler on linux

[bhatsach]

This change is needed to compile pluginlib package and packages which
depend on it on linux clang compiler

[bhatsach]

Points to note

• I am adding a nightly job on ROS2 CI to build on linux clang compiler in order to detect bugs early. Mac OS X builds can take very long to complete. Here's the PR link for the same: ros2/ci#244

Testing done

• I have tested the above changes by doing a colcon build locally with all the ros2 packages pulled

[tfoote]

This approach seems like it's trying to fix the test and not to identify the more general underlying requirement. What is the differentiating factor that requires this flag being added? Can you enumerate the cases, because at a glance at this looks like it should just be detecting linux an adding the additional compile flag? The previous flag was effectively selecting linux as osx runs clang by default and windows is vsbuild.

[bhatsach]

This approach seems like it's trying to fix the test and not to identify the more general underlying requirement. What is the differentiating factor that requires this flag being added? Can you enumerate the cases, because at a glance at this looks like it should just be detecting linux an adding the additional compile flag? The previous flag was effectively selecting linux as osx runs clang by default and windows is vsbuild.

Makes sense, I may be able to just update the check to verify that we are on linux. Will look into this now.

[bhatsach]

Updated the PR with just a check for linux for linking experimental filesystem libraries. Also updated the check to linux while exporting the libraries. Experimental Filesystem is a linux specific concept rather than compiler specific.

[bhatsach]

@nuclearsandwich and @tfoote, please let me know if you have any comments on the latest revision, thanks

[nuclearsandwich]

One question about the CMake logic for long term *nix support. Once that is settled we'll need to run CI.

[nuclearsandwich]

• Linux
• Linux-aarch64
• macOS
• Windows
• Linux Clang

[nuclearsandwich]

Will merge as long as CI comes back green.

[bhatsach]

One question about the CMake logic for long term *nix support. Once that is settled we'll need to run CI.

Thanks a lot for identifying and updating this. I locally tested it on ubuntu and the change looks good :)

[nuclearsandwich]

@bhatsach do you have any insight into the warnings that are coming back on the Linux Clang job?

[nuclearsandwich]

Because the warnings are in other packages and don't seem directly related to pluglinlib and the supported builds have no errors. I'm going to merge with the expectation that those interested in getting the clang on Linux builds fully green will do so.