How to make qtcreator work with c++11

I am trying to make school project work locally, but I keep having trouble with qtcreator and c++11.

The error message keeps saying they can't use c++11 stuffs, such as

/Users/yoong93/Qt5.6.2/5.6/clang_64/lib/QtCore.framework/Headers/qstring.h:
757: error: no type named 'u16string' in namespace 'std'
    inline std::u16string toStdU16String() const;

       ~~~~~^

And the .pro file already have following lines --

QMAKE_CXXFLAGS += -std=c++14
CONFIG += c++14
unix:!macx {
    LIBS += -lGLU
}
macx {
    QMAKE_CFLAGS_X86_64 += -mmacosx-version-min=10.7
    QMAKE_CXXFLAGS_X86_64 = $$QMAKE_CFLAGS_X86_64
    CONFIG +=c++11
}

So I expected there is nothing wrong with the project file, which is the most common issue that others had. I have no clue where it went wrong. Does anyone know about the issue? I am using OSX El Captain version 10.11.6, and QT 5.6.1, and I have Xcode version 7.3.1 installed. Thanks in advance.