title: Cmake in QtCreator: Source file not displayed in project file tree view
body: <p>I'm using CMake och QtCreator to write C code. I open my project by opening the CMakeLists.txt file on the top level.
  I see all the files in the project file tree view in QtCreator that are used to build executables or libraries like this:</p>
  
  <pre><code>add_library( my_lib file1.c )
  add_executable( my_executable file2.c )
  </code></pre>
  
  <p>But I have one file that are used in precompilation for postgres like this:</p>
  
  <pre><code>set( MY_CMD "/usr/pgsql-9.3/bin/ecpg" )
  set( MY_ARG "file3.pgc" )
  add_custom_target( dummy_target ALL COMMAND ${MY_CMD} ${MY_ARG} WORKING_DIRECTORY ${MY_DIR} )
  </code></pre>
  
  <p>The file file3.pgc is not displayed in the project file tree view. How can I make QtCreator find that file?</p>