Update README for newly added configure switches

Update README for newly added configure switches

Add --with-gssapi*, --with-libintl*, and --with-roken for the new
library location flags.  Add --with-docbook2pdf and
--with-docbook-stylesheets for the automatic discovery of tools for
building the documentation.  Expand and correct the descriptions of
--with-html-xml and --with-xslt-processor.

Change-Id: Ibe0a94ce3e9c6393bde009bf08f8b9dfac6b84a6
Reviewed-on: http://gerrit.openafs.org/5092
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>