title: Remove specific file from cmake build
body: <p>I have a project in which i have essentially two main methods. One for testing and one for, well, running the code. 
  Normally you would create submodules, but this is not an option. </p>
  
  <pre><code>file(GLOB sources "*.cpp")
  file(GLOB headers "*.h")
  add_executable(testing ${sources} ${headers})   
  add_executable(main ${sources} ${headers})   
  </code></pre>
  
  <p>So testing should compile all sources except for main.cpp.
  Main should compile everything but testing.cpp. </p>