opentracing-shim target includes are missing when opentelemetry-cpp is added as a subdirectory to a larger project

[andremarianiello]

Describe your environment Describe any aspect of your environment relevant to the problem, including your platform, build system, version numbers of installed dependencies, etc. If you're reporting a problem with a specific version of a library in this repo, please check whether the problem has been fixed on main branch.

Rocky Linux 8, cmake version 3.27.7

Steps to reproduce

1. Create a cmake project (/myproject) that adds opentelemetry-cpp as a subdirectory, either with add_subdirectory or FetchContent_MakeAvailable
2. Add opentelemetry_opentracing_shim as a link to a local target in your project
3. Try to build your local target

What is the expected behavior?
I expect that the opentelemetry_opentracing_shim include paths point to the location of the third_party/opentracing-cpp header files

What is the actual behavior?
The include paths are relative to my local project's root, not the opentelemetry-cpp root. That is, I see paths like /myproject/third_party/opentracing-cpp/include instead of /myproject/opentelemetry-cpp/third_party/opentracing-cpp/include. This causes errors about missing headers.

Additional context
The root cause appears to be in opentracing-shim/CMakeLists.txt. It is added some include dirs to the shim target, but they are relative to CMAKE_BINARY_DIR/CMAKE_SORUCE_DIR. This works when opentelemetry-cpp is the top-level project, but in this case myproject is at the top-level, so these paths are no longer correct. I think making these paths relative to PROJECT_BINARY_DIR/PROJECT_SOURCE_DIR would be better since the paths would always be constructed relative to the actual opentelemetry-cpp directory.