Add SYSTEM to target_include_directories in vendored CMake wrappers and
set INTERFACE_SYSTEM_INCLUDE_DIRECTORIES on imported / sub-project
targets so warnings from third-party headers no longer surface in our
build output. Introduces a mark_target_includes_system() helper in
cmake/utils.cmake (resolves aliases, skips missing targets) used by the
upstream-add_subdirectory wrappers (glog, gflags, googletest, yaml-cpp,
protobuf, antlr).
51c6d9e0ec