Try using -Wno-documentation-deprecated-sync on underlying targets, not aliases

Author: raulcd

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -3414,16 +3414,15 @@ function(build_google_cloud_cpp_storage)
 
   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}")
+  # Suppress documentation warnings for google-cloud-cpp headers.
+  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|AppleClang")
+    foreach(_target storage_client rest_internal common)
+      if(TARGET ${_target})
+        target_compile_options(${_target} INTERFACE -Wno-documentation
+                                                    -Wno-documentation-deprecated-sync)
       endif()
-    endif()
-  endforeach()
+    endforeach()
+  endif()
 
   # Remove unused directories to save build directory storage.
   # 141MB -> 79MB