Add NUT version number into footer of HTML man pages

Override AsciiDoc default for footer-txt to include NUT version number
into footer of HTML man pages. This commit addresses the 2nd point of
networkupstools/nut#150
Also check for source-highlight at configure time

source-highlight is used for documentation generation. It's
however optional, so we just check for the sake of completion
Explicitly use subdir-objects in automake init

Closes networkupstools/nut#155
Don't reference subdir-object with $(top_srcdir)

Replace references to objects in separate directories that were
using $(top_srcdir) by the expanded version '../../'. The variable was
otherwise part of the path, resulting in build failures. This completes
commit f8abb9b

Closes networkupstools/nut#155
Clarify a bit more Avahi build requirements
docs: add NUT version number/date into footer of HTML pages

Reference: https://github.com/networkupstools/nut/issues/150
docs: move DocBook options common to html stylesheets to common.xsl
docs: add NUT version number/date in PDF documents

Reference: https://github.com/networkupstools/nut/issues/150