warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]

I'm using Qt5 with Clang on Debian Jessie. To experiment with generic lambdas, in the .pro file there is:

CONFIG += c++14

And after building I got:

warning: 'auto' type specifier is a C++11 extension [-Wc++11-extensions]

To get rid of this obvious message I did:

QMAKE_CXXFLAGS += -Wc++11-extensions

But I keep getting the obvious message. Why? How to hide it?