Avoid a problem when clang pretends to be GNUC
diff --git a/include/grpc++/config.h b/include/grpc++/config.h
index e10f00f..8ef5d71 100644
--- a/include/grpc++/config.h
+++ b/include/grpc++/config.h
@@ -65,11 +65,13 @@
   ::google::protobuf::io::ZeroCopyInputStream
 #endif
 
+#ifndef __clang__
 #ifdef __GNUC__
 #if (__GNUC__ * 100 + __GNUC_MINOR__ < 406)
 #define GRPC_NO_NULLPTR
 #endif
 #endif
+#endif
 
 #ifdef GRPC_NO_NULLPTR
 #include <memory>