16.04 uses gcc 5.4 by default. You’ll need to install gcc 4.9 and configure OpenCV to use 4.9 instead:
sudo apt-get install g++-4.9 cmake -DCMAKE_C_COMPILER=/usr/bin/gcc-4.9 -DCMAKE_CXX_COMPILER=/usr/bin/g++-4.9 .
If you have CUDA installed you may want to disable compiling the CUDA libraries as well, or else suffer another hour+ of compilation time. Add -DWITH_CUDA=OFF to disable CUDA.