Almost working out-of-tree builds for the documentation,

Almost working out-of-tree builds for the documentation,
with a few hacks right now:
- The tip.png, warning.png and note.png images are missing from the
  pdfs and I have no idea how to tell fop how to find them.
- Disabling/enabling building the guides via option currently doesn't
  work (probably too many macros :-), so comment out the subdir instead.
- Right now, in order to build the devleopers guide we need to do the
  following in the source docbook directory:
  touch wsdg_graphics/toolbar/dummy.dummy

Apart from these: The build works with a pristine docbook dir
(svn status --no-ignore).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@32004 f5534014-38df-0310-8fa8-9805f1628bb7