title: Detect CMake build target in CMakeLists.txt?
body: <p>In CMake script <code>CMakeLists.txt</code>, how to detect the current build target, like the parameter passed to cmake command via <code>-A</code> option? There is variable <code>WIN32</code> to detect Windows, but still not the parameter for <code>-A</code> option, like <code>-A X64</code>.</p>
  
  <pre><code>if (WIN32)
      #do something
  endif (WIN32)
  </code></pre>