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.

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>
(cherry picked from commit cc2bc3e17ff5f7a10c515e309f8fec47a6fa14b6)

Change-Id: I4773affa193b5210da6416dd8b4321bd3f471429
Reviewed-on: http://gerrit.openafs.org/4794
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>