title: how to build opencv for python3 when both python2 and python3 are installed
body: <p>I was trying to build opencv for python3. However, cmake always sets python build option to be python2.7.11 even after I manually specified include and lib option for python3:</p>
  
  <pre><code>    --   Python 2:
      --   Interpreter:                 /home/ryu/anaconda2/bin/python2.7 (ver 2.7.11)
  
      --   Python 3:
      --     Interpreter:                 /usr/bin/python3 (ver 3.4.3)
      --     Libraries:                   /usr/lib/x86_64-linux-gnu/libpython3.4m (ver 3.4.3)
      --     numpy:                       /home/ryu/.local/lib/python3.4/site-packages/numpy/core/include (ver 1.11.0)  
  
      --  packages path:               lib/python3.4/dist-packages
  
  -- 
  --   **Python (for build):            /home/ryu/anaconda2/bin/python2.7**
  </code></pre>
  
  <p>Did I miss some cmake option?</p>
  
  <p>OS: Ubuntu 14,04</p>
  
  <p>thanks</p>