Details:
- Implemented an option (-i LIST) to gen-make-frag.sh that allows the
  caller to optionally ignore additional directories when walking the
  source directory. (Note that previously the standard -- and only --
  way to ignore directories was to add them to the ignore_list file,
  which is a required argument to the script.)
- I implemented this feature for something but then ended up not needing
  it, but figured it might be helpful in the future.