doc: let configure find the XML tools if possible

doc: let configure find the XML tools if possible

configure should attempt to find the XML tools we need to process
the documentation.  if it can't, it should provide a safe default.
still allow the user to override via command line.

Change-Id: Ifaf3a5b090e93858c3d3c88363760c508030db90
Change-Id: Ib9558c37b04a4533e91b172a2a62039e4ed06d6f
Reviewed-on: http://gerrit.openafs.org/4766
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>