title: building opencv, got undefined reference to avformat_network_init error
body: <p>I am using CentOS6.5, I downloaded opencv-2.4.9, made build directory, and under there, ran 'cmake ..' and 'make'. During the make run, I got this error message.</p>
  
  <p><img src="https://i.stack.imgur.com/Ve5qU.jpg" alt="link error message"></p>
  
  <p>I found avformat_network_init in ffmpeg source, I installed ffmpeg once in my home and then to /usr/lib.  but I don't know how to tell cmake where ffmpeg is installed (include header and library).<br>
  Now I see some *.so files from ffmpeg under /usr/lib64. but why can't cmake find the ffmpeg location?
  Thanks in advance.</p>