title: "In-source builds are not allowed" in cmake
body: <p>I'm new to cmake, and I'm only using it to install opencv on my ubuntu linux.
  Here's the command I ran: <code>cmake -DCMAKE_BUILD_TYPE=Release DCMAKE_INSTALL_PREFIX=/home/jinha/OCV/source</code></p>
  <p>Then it returns the error:</p>
  <pre><code>FATAL: In-source builds are not allowed. You should create separate directory for build files.
  </code></pre>
  <p>My current directory, <code>~/OCV/build/opencv</code>, does contain the CMakefiles.txt file, so that's not the problem. I tried to change the directory in my command, but they all raise the same error. I saw the other answers on this issue, so I erased <code>CMakeFiles</code> directory and <code>CMakeCache.txt</code> file every time before I ran the command, but none of them worked.<br />
  Thanks.</p>