Set default X11 resources dir to $prefix/lib/X11/app-defaults.

Previously, X11 resources for gxditview were installed in
/usr/X11/lib/X11/app-defaults, no matter which prefix was set.  Also,
the `GROFF_APPRESDIR_DEFAULT' m4 macro had a bug (failed to detect
properly the location of app-defaults on the system).

Now, gxditview resources are installed in
appresdir=$prefix/lib/X11/app-defaults.  If `appresdir' is not a
standard X11 resource directory, the environment variable
`XFILESEARCHPATH' should be set to this path.

The behavious of --with-appresdir is unchanded: this option can still
be used to set appresdir, and its value will not be modified by the
prefix.
Add ChangeLog entry for previous commit on X11 resources update.
Fix last ChangeLog entry.