title: fatal error: opencv2/gpu/gpu.hpp: No such file or directory opencv and GPU installation
body: <p>the package I am trying to install is in the below link 
  <a href="https://github.com/wanglimin/dense_flow" rel="nofollow noreferrer">https://github.com/wanglimin/dense_flow</a> </p>
  
  <pre><code>  (cv) -group-1-3p16:~/dense_flow$ sudo make 
      Scanning dependencies of target denseFlow_gpu
      [ 50%] Building CXX object CMakeFiles/denseFlow_gpu.dir/denseFlow_gpu.cpp.o
      /home/vinsent_research/dense_flow/denseFlow_gpu.cpp:4:31: fatal error: opencv2/gpu/gpu.hpp: No such file or directory
      compilation terminated.
      CMakeFiles/denseFlow_gpu.dir/build.make:62: recipe for target 'CMakeFiles/denseFlow_gpu.dir/denseFlow_gpu.cpp.o' failed
      make[2]: *** [CMakeFiles/denseFlow_gpu.dir/denseFlow_gpu.cpp.o] Error 1
      CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/denseFlow_gpu.dir/all' failed
      make[2]: *** [CMakeFiles/denseFlow_gpu.dir/all] Error 2
      Makefile:83: recipe for target 'all' failed
      make: *** [all] Error 2
  Not sure how to resolve this issue, I have installed opencv 3.4.1 on ubuntu 18, the installation was successfull, followed these post 
  
  [https://github.com/VinACE/Install-TensorFlow-OpenCV-GPU-Ubuntu-17.10][2] 
  
  
  my cmake 
  
      sudo cmake -D CMAKE_BUILD_TYPE=RELEASE \
          -D CMAKE_INSTALL_PREFIX=/usr/local \
          -D INSTALL_PYTHON_EXAMPLES=ON \
          -D INSTALL_C_EXAMPLES=OFF \
          -DPYTHON2_EXECUTABLE=/usr/bin/python2 \
          -D OPENCV_EXTRA_MODULES_PATH=~/opencv_contrib-3.4.1/modules \
          -D PYTHON_EXECUTABLE=~/.virtualenvs/cv/bin/python \
          -D WITH_CUDA=ON \
          -D CMAKE_CXX_COMPILER:FILEPATH=/usr/bin/g++-5 \
          -D CMAKE_C_COMPILER:FILEPATH=/usr/bin/gcc-5 \
          -D BUILD_EXAMPLES=ON ..
  </code></pre>
  
  <p><a href="http://%20%20http://%20https://github.com/VinACE/Install-TensorFlow-OpenCV-GPU-Ubuntu-17.10" rel="nofollow noreferrer">the ubuntu opencv 3.4.1 install what i have followed </a></p>