Isolate Clang-only CMake flags (#15811)

Author: vkucera

dependencies/O2CompileFlags.cmake

@@ -108,8 +108,12 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
                 WARNINGS ${O2_NEW_COMPILER_WARNINGS_NO_ERROR})
 endif()
 
-string(JOIN " " CMAKE_C_WARNINGS "-Wno-unknown-warning-option" "-Wno-vla-cxx-extension" ${O2_C_ENABLED_WARNINGS} ${O2_C_ENABLED_WARNINGS_NO_ERROR} ${O2_NEW_COMPILER_NO_ERROR_FLAGS})
-string(JOIN " " CMAKE_CXX_WARNINGS "-Wno-unknown-warning-option" "-Wno-vla-cxx-extension" ${O2_CXX_ENABLED_WARNINGS} ${O2_CXX_ENABLED_WARNINGS_NO_ERROR} ${O2_NEW_COMPILER_NO_ERROR_FLAGS})
+string(JOIN " " CMAKE_C_WARNINGS ${O2_C_ENABLED_WARNINGS} ${O2_C_ENABLED_WARNINGS_NO_ERROR} ${O2_NEW_COMPILER_NO_ERROR_FLAGS})
+string(JOIN " " CMAKE_CXX_WARNINGS ${O2_CXX_ENABLED_WARNINGS} ${O2_CXX_ENABLED_WARNINGS_NO_ERROR} ${O2_NEW_COMPILER_NO_ERROR_FLAGS})
+if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+  string(APPEND CMAKE_C_WARNINGS " -Wno-unknown-warning-option -Wno-vla-cxx-extension")
+  string(APPEND CMAKE_CXX_WARNINGS " -Wno-unknown-warning-option -Wno-vla-cxx-extension")
+endif()
 
 string(REGEX MATCH "-O[0-9]+" CMAKE_FLAGS_OPT_VALUE "${CMAKE_CXX_FLAGS}")
 if(NOT CMAKE_FLAGS_OPT_VALUE OR CMAKE_FLAGS_OPT_VALUE STREQUAL "-O0" OR CMAKE_FLAGS_OPT_VALUE STREQUAL "-O1")