doxygen: make dox

Add an optional make target (make dox) and doxygen configuration to
generate doxygen output files.  Auto-detect when the doxygen and
graphviz dot tools are available.  When dot is present, configure
doxygen to create dependency graphs.

Since the graph generation can take a very long time, a new
configure option has been added to override the dot tool
auto-detection. To disable the graph generation (even if dot is
installed), run configure with the option: --without-dot

When graph generation is desired, but graphviz dot is not present in
the PATH, specify the path to dot with the configure option
--with-dot=<path-to-dot>.

The configure summary has been updated to show when doxygen document
and graph generation is configured.

Thank you Jason Edgecombe for providing the doxygen configuration
for OpenAFS.

Change-Id: Ie875fc2961877ee76e4c17631bbb95c29ef20b9e
Reviewed-on: http://gerrit.openafs.org/10970
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>