docs: Support building pdfs from markdown using pandoc

The Xen command line parameters document is far more useful as an indexed pdf
than it is as unindexed html webpage.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- reran autogen.sh ]

config/Docs.mk.in

@@ -11,4 +11,5 @@ POD2MAN             := @POD2MAN@
 POD2HTML            := @POD2HTML@
 POD2TEXT            := @POD2TEXT@
 MARKDOWN            := @MARKDOWN@
+PANDOC              := @PANDOC@
 PERL                := @PERL@