Unable to add below statements in code QMAKE_CXXFLAGS += -std=c++14

I am using chrono library for calculating elapsed time but I am getting below warning.

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

I am beginner in C++ and don't know how to add QMAKE_CXXFLAGS += -std=c++14.

Any help would be highly appreciated.