Add --doc= configure option to disable helpfile generation
It will get disabled automatically if the deps (xmlto and xsltproc)
are missing
Also added checks for asciidoc (a2x) for the skype plugin, which needs
it for the skyped man page, and will also get that disabled if --doc=0
is passed or if asciidoc isn't installed.
This should keep those ugly deps under control for now.
I'd like to replace them at some point with something less dumb.
