title: CMake cannot locate io.h
body: <p>I'm trying to install OpenCV 2.4.9 on Ubuntu 18.04. I'm using this command:</p>
  
  <p><code>cmake -D BUILD_TIFF=ON -D WITH_CUDA=OFF -D ENABLE_AVX=OFF -D WITH_OPENGL=OFF -D WITH_OPENCL=OFF -D WITH_IPP=OFF -D WITH_TBB=ON -D BUILD_TBB=ON -D WITH_EIGEN=OFF -D WITH_V4L=OFF -D WITH_VTK=OFF -D BUILD_TESTS=OFF -D BUILD_PERF_TESTS=OFF -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local /opt/opencv-2.4.9/</code></p>
  
  <p>But it says <code>Configuring incomplete, errors occurred!</code></p>
  
  <p>When I checked <code>CMakeError.log</code> get these two error messages:</p>
  
  <p>1) <code>/opt/opencv-2.4.9/release/CMakeFiles/CMakeTmp/CheckIncludeFile.c:1:10: fatal error: io.h: No such file or directory</code></p>
  
  <p>2) <code>/opt/opencv-2.4.9/release/CMakeFiles/CMakeTmp/CheckIncludeFile.c:1:10: fatal error: ffmpeg/avformat.h: No such file or directory</code></p>
  
  <p>I'm able to locate numerous <code>io.h</code> files in my OS and <code>avformat.h</code> is also there. I guess I'm missing some path setting. Please suggest.</p>