Feed Clang a pragma to ignore -Wc++11-extensions to let it build Skia on Mac.

include/core/SkPostConfig.h

Index: include/core/SkPostConfig.h
diff --git a/include/core/SkPostConfig.h b/include/core/SkPostConfig.h
index 86393ed446103fbd927bb10d84742d91dec5ff59..64768c6faf82ca763080ce0a22586360b1eebae6 100644
--- a/include/core/SkPostConfig.h
+++ b/include/core/SkPostConfig.h
@@ -322,6 +322,11 @@
     #if defined(_MSC_VER)
         #define SK_OVERRIDE override
     #elif defined(__clang__) && !defined(SK_BUILD_FOR_IOS)
+        // Clang defaults to C++03 and warns about using override. Squelch that. Intentionally no
+        // push/pop here so all users of SK_OVERRIDE ignore the warning too. This is like passing
+        // -Wno-c++11-extensions, except that GCC won't die (because it won't see this pragma).
+        #pragma clang diagnostic ignored "-Wc++11-extensions"
+
         #if __has_feature(cxx_override_control)
             // Some documentation suggests we should be using __attribute__((override)),
             // but it doesn't work.