qtserialport... error: ‘Q_NULLPTR’ was not declared in this scope

Am trying to install qt creator following these sequences suggested by a hardware provider...

sudo apt-get install libudev-dev
git clone git://code.qt.io/qt/qtserialport.git
cd qtserialport
git checkout origin/old/5.2
cd .. 
mkdir qtserialport-build 
cd qtserialport-build 
qmake ../qtserialport/qtserialport.pro 
make 
sudo make install

cd ../qtserialport/
git checkout origin/5.3
cd ../qtserialport-build/
make

But am getting errors at the last make:

In file included from /home/polar/soft/lib/qt/qtserialport/src/serialport/qserialport.cpp:45:0:
/home/polar/soft/lib/qt/qtserialport/src/serialport/qserialport.h:190:44: 

error: ‘Q_NULLPTR’ was not declared in this scope
     explicit QSerialPort(QObject *parent = Q_NULLPTR);
                                            ^

On stackoverflow some suggested to add QMAKE_CXXFLAGS += -std=c++0x, others CONFIG += c++11 to the .pro file. These did not work for me.... In fact there are at least two .pro files... which one I need amendment?

Any suggestions please?