[Groff] gxditview transition done

[Werner LEMBERG]

I've now finished the integration of gxditview into groff's standard
directory hierarchy (to be found now in src/devices/xditview,
src/utils/xtotroff, and src/libs/libxutil).  It was surprisingly much
work.

GXditview.ad isn't yet installed; a man page for xtotroff (which is
now installed also) is still missing.

Please test.  The devX... directories are now installed on demand
(this is, if gxditview is installed).  gxditview won't be installed on
platforms with no X11 or if configure's --without-x flag is given.


    Werner