Don't hardcode docbook XSL namespace URL

Docbook XSL got updated to version 1.79.1 in NixOS/nixpkgs@fb893a8 and
we're still referring to the hardcoded previous version.

So instead of just updating this to 1.79.1 we're going to use "current"
in the hope that this won't happen again.

I have tested this by building the manual under Nix(OS) but I haven't
tested this in a non-Nix environment, so I'm not sure whether this could
have implications.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @edolstra

doc/manual/local.mk

@@ -10,7 +10,7 @@ XSLTPROC = $(xsltproc) --nonet $(xmlflags) \
   --stringparam generate.toc "book toc" \
   --param keep.relative.image.uris 0
 
-docbookxsl = http://docbook.sourceforge.net/release/xsl-ns/1.78.1
+docbookxsl = http://docbook.sourceforge.net/release/xsl-ns/current
 docbookrng = http://docbook.org/xml/5.0/rng/docbook.rng
 
 MANUAL_SRCS := $(call rwildcard, $(d), *.xml)