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I realized what the problem was. I installed qt4-dev-tools (instead of qt5-dev-tools) that are in collision with current Qt version. As the @berak suggested , I deleted OpenCV with :

sudo find / -name "opencv" -exec rm -i {} \;

Then installed qt5 dev tools: sudo apt-get install qt5-dev-tools

I also should've add this line in .pro file :

LIBS += -lX11 -ldl -lXext -lz

and rebuild OpenCV (WITH_OPENGL in CMake-gui wasn't checked).