Add support and documentation for out-of-tree builds with syntax
like the Linux kernel (make O=<dir>).

'default n' is the default, so there's no need to say it explicitly.

  TODO: document HAVE_MANPAGES and HAVE_INFOPAGES (and adjust line-numbers in the text accordingly)

- Add some more explanation to the -clean and -dirclean targets

  Let's see if the date property works..

  Thanks to Thomas Lundquist for pointing this out.

  Silly, unchecked sed -i -e "/[^b]zcat/s/zcat/\$\(ZCAT\)/g" $(svngrep "[^b]zcat" * -rl | grep -v Config.in)