title: Cannot set CMAKE_PREFIX_PATH in ccmake or cmake-gui
body: <p>I want to use <code>CMAKE_PREFIX_PATH</code> to set a directory (say <code>/usr/local2</code>) for <code>cmake</code> to search for headers and libraries. I tried to use the command line</p>
  
  <pre><code>cmake -DCMAKE_PREFIX_PATH=/usr/local2 /path/to/src
  </code></pre>
  
  <p>But <code>cmake</code> does not seem to take that hint (i.e. it still found the library in system path <code>/usr/local</code>). I also tried <code>ccmake</code> and <code>cmake-gui</code>, but neither provide me the <code>CMAKE_PREFIX_PATH</code> entry to edit. When I search for "PREFIX", the only one shows up is <code>CMAKE_INSTALL_PREFIX</code>. Am I not looking at the right place? Or did I miss something when installing <code>cmake</code>?</p>
  
  <p>I am using <code>cmake</code> version 3.1.3 (and also tried 3.0.2), which is installed with MacPorts. And I did remember to check the "advanced" box for all options.</p>
  
  <p>My <code>CMakeLists.txt</code> file is pretty simple</p>
  
  <pre><code>cmake_minimum_required (VERSION 3.0)
  project (my_project CXX)
  find_package (PNG REQUIRED)
  include_directories (${PNG_INCLUDE_DIRS})
  add_executable (my_executable test.cc)
  target_link_libraries (my_executable ${PNG_LIBRARIES})
  </code></pre>
  
  <p>and I also made sure the <code>/usr/local2/lib</code> contains the <code>libpng.dylib</code> for <code>cmake</code> to search for.</p>
  
  <p>In the <code>CMakeCache.txt</code> file, I found</p>
  
  <pre><code>//No help, variable specified on the command line.
  CMAKE_PREFIX_PATH:UNINITIALIZED=/usr/local2
  </code></pre>
  
  <p>Thanks in advance for your help!</p>