title: Generating RDoc fails with Directory .ext/rdoc already exists
body: <p>I'm just installing Ruby Enterprise and cannot change the output directory for this crashing command called by Ruby Enterprise installer.</p>
  
  <p>So how do I delete the directory to satisfy this exception:</p>
  
  <p>Generating RDoc documentation
  ./miniruby -I./lib -I.ext/common -I./- -r./ext/purelib.rb  ./runruby.rb --extout=.ext  -- "./bin/rdoc" --all --ri --op ".ext/rdoc" "."</p>
  
  <p>Directory .ext/rdoc already exists, but it looks like it
  isn't an RDoc directory. Because RDoc doesn't want to risk
  destroying any of your existing files, you'll need to
  specify a different output directory name (using the
  --op  option).</p>