CMake: Make it easier to generate the release notes & NEWS.

Add convenience targets for generating the release notes and the NEWS
file. Make sure we don't run multiple instances of a2x + AsciiDoc at the
same time.

Add the docbook directory to the build by default unless we're running
Windows. Explain why we don't yet build docs on Windows. Make each
docbook makefile target optional.

Split the ENABLE_GUIDES option into ENABLE_HTML_GUIDES and
ENABLE_PDF_GUIDES. Add a default "all_guides" target if either is on.
Remove the Debian patch that hacked around the PDF requirement.

Copy ws.css to the docbook build directory. Don't build PDF release
notes. I'm not sure we ever used them and I don't want to install Java
and FOP just to make a release.

Change-Id: Ia2f710000c17f9e0b4b514fd373d9a5902889553
Reviewed-on: https://code.wireshark.org/review/5712
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>