title: Homebrew: Could not symlink, /usr/local/share/man/man7 is not writable
body: <p>I'm installing LightGBM on OSX, following the <a href="https://github.com/Microsoft/LightGBM/wiki/Installation-Guide" rel="noreferrer">installation guide</a>. While installing cmake, <code>Homebrew</code> gives such warning message:</p>
  
  <pre><code>Warning: cmake-3.8.1 already installed, it's just not linked.
  </code></pre>
  
  <p>Then I try:</p>
  
  <pre><code>$brew link cmake
  </code></pre>
  
  <p>it gives:</p>
  
  <pre><code>Linking /usr/local/Cellar/cmake/3.8.1...
  Error: Could not symlink share/man/man7/cmake-buildsystem.7
  /usr/local/share/man/man7 is not writable.
  </code></pre>