title: Group build targets in CMake project
body: <p>I just included doxygen generation in my project using the <em>add_custom_target()</em> command. Now I did not include ALL as I don't want this build be default.</p>
  
  <p>Now I have the following scenario.</p>
  
  <p>project/subproj1/doc
  project/subproj2/doc</p>
  
  <p>In each subproject there is a CMakeLists.txt file with:<code>
  add_custom_target(gen_doc_subproj1 ...)
  add_custom_target(gen_doc_subproj2 ...)</code></p>
  
  <p>What I'd like to achieve is that after generating my make files I can type:</p>
  
  <p><strong>make doc</strong>
  and it will build all documentation targets.</p>
  
  <p>Is there some construction in CMake such as:<code>
  if_not_exist_target_group(doc)
      create_target_group(doc)
      endif
      add_to_target_group(gen_doc_subproj1, doc)</code></p>
  
  <p>Any pointers would be appriciated.</p>