title: Specify explicit header files in CMake
body: <p>I have precompiled header file stdafx.h which is specific to each target. My CMake project roughly looks like below:</p>
  
  <pre><code>include_directories(&lt;prefix&gt;/include_a)
  include_directories(&lt;prefix&gt;/include_b)  #&lt;---- contains stdafx.h that should NOT be used
  include_directories(&lt;project_dir&gt;/)  #&lt;----- contains stdafx.h that should be used
  
  add_executable(&lt;project_name&gt; &lt;sources&gt;)
  </code></pre>
  
  <p>Problem: I want to exclude precompiled header that comes as part of <code>&lt;prefix&gt;/include_b</code> directory. If thats not possible then specify header files explicitly. I know <code>gcc</code> supports <code>-include</code> option but is there any cleaner option with CMake that fits my requirement?</p>