Add mixin to force clang and libcxx

[emersonknapp]

Edited up to date summary:

• Adds a mixin that performs the build using clang6, with LLVM's libcxx instead of gnu libstdc++
• Adding some CMake variables to force building some vendor packages

This mixin would be used on Clang CI build to enable Thread Safety Analysis, and generally validate that ROS2 works with libcxx

Related to:

• Pass through c++ build flags ros2/console_bridge_vendor#4
• Pass through compiler and c++ flags ros2/yaml_cpp_vendor#4
• Dynamically choose the appropriate library for experimental filesystem ros/pluginlib#146
• Pass through cxx flags ros2/tinyxml_vendor#12
• Pass through CXX flags to OGRE vendor build ros2/rviz#381
• Enable forcing build from source, and pass through c++ flags ros2/poco_vendor#24
• Don't hardcode int64_t for duration type representations ros2/rclcpp#648

Connects to ros2/ros2#664

[thomas-moulard]

Could you elaborate when this global flags is necessary? compared to e.g. using CXXFLAGS?

[emersonknapp]

@thomas-moulard here is an example that uses both flags ros2/poco_vendor#24

[emersonknapp]

I was able to "force libcxx" down the line by using full passthrough of CMAKE_CXX_FLAGS, as per @dirk-thomas suggestion. This seems to have worked for all downstream packages. As for FORCE_BUILD_ALL_VENDOR, there were only two existing vendor packages that didn't build by default, so I went in favor of specificity

[emersonknapp]

With this mixin, plus a few other minor pull requests that are incoming, colcon build fully succeeds and I get the following colcon test results. I think that starting from here it would be a meaningful CI build.

Summary: 231 packages finished [10min 8s]
  1 package failed: sros2
  13 packages had stderr output: class_loader composition demo_nodes_cpp demo_nodes_cpp_native image_tools intra_process_demo logging_demo osrf_pycommon pendulum_control pluginlib qt_gui_cpp rosidl_parser rqt
  11 packages had test failures: class_loader composition demo_nodes_cpp demo_nodes_cpp_native image_tools intra_process_demo logging_demo pendulum_control pluginlib qt_gui_cpp rosidl_parser

[nuclearsandwich]

I think that starting from here it would be a meaningful CI build.

@emersonknapp are you planning to switch our existing linux-clang job to use libcxx or add an additional job?

[nuclearsandwich]

Full CI on a clang+libcxx on Linux run

• Linux Clang+libcxx

[emersonknapp]

I think that starting from here it would be a meaningful CI build.

@emersonknapp are you planning to switch our existing linux-clang job to use libcxx or add an additional job?

The way I see it, there is value in having CI builds for both:

• Linux Clang with libstdc++
• Linux Clang with libc++