title: Run Doxygen only on select files / modules?
body: <p>Recently I started writing some doxygen docs in an existing project which already has quite a lot of doxygen comments.</p>
  
  <p>Since I'm learning a bit - I like to iterate with making edits and generating docs, since doc generation is quite slow - 3-5min. This becomes un-workable.</p>
  
  <p>I managed by deleting most of the files in the source tree so doxy only found the ones I was editing but this is really a horrible solution and not something I'd want to do frequently.</p>
  
  <p>Is there a way (command line arg or env variable for eg) - to limit which files/modules are used for generating docs - so rebuilding docs can be done much faster?</p>