C++ standard library change in official builds

[Lizan Zhou]

Hey folks,

As a heads up, with #8859 we switched the C++ standard library to libc++ from LLVM from libstdc++ in our official builds, as discussed in #4251. Building with libstdc++ is still supported and tested in CI, see the instructions for how to selecting libc++ or libstdc++.

If you're using dynamic OpenTracing tracers, you have to make sure your opentracing dynamic tracer is also built with same C++ standard library that Envoy is built with. This means you have to rebuild the OpenTracing tracers or Envoy.

If you have your own build without using our CI script by invoking Bazel directly, this change won't affect your build. See the instructions how to use libc++ in your build.

Let us know by opening GitHub issues if you see any problems.

Thanks,

Lizan