Try using INTERFACE_SYSTEM_INCLUDE_DIRECTORIES so documentation warnings doesn't error

Author: raulcd

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -3412,13 +3412,19 @@ function(build_google_cloud_cpp_storage)
   # Unity build causes some build errors.
   set(CMAKE_UNITY_BUILD FALSE)
 
-  # Disable -Wdocumentation-deprecated-sync warning for google-cloud-cpp.
-  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-    string(APPEND CMAKE_CXX_FLAGS " -Wno-documentation-deprecated-sync")
-  endif()
-
   fetchcontent_makeavailable(google_cloud_cpp)
 
+  # Mark google-cloud-cpp includes as SYSTEM to suppress documentation warnings.
+  foreach(_target storage_client storage_protos rest_internal common)
+    if(TARGET ${_target})
+      get_target_property(_include_dirs ${_target} INTERFACE_INCLUDE_DIRECTORIES)
+      if(_include_dirs)
+        set_target_properties(${_target} PROPERTIES INTERFACE_SYSTEM_INCLUDE_DIRECTORIES
+                                                    "${_include_dirs}")
+      endif()
+    endif()
+  endforeach()
+
   # Remove unused directories to save build directory storage.
   # 141MB -> 79MB
   file(REMOVE_RECURSE "${google_cloud_cpp_SOURCE_DIR}/ci")