[ESS] make problems with latest git

[Ross Boylan]

On Tue, Mar 06, 2018 at 10:18:13AM -0800, Ross Boylan wrote:
...
> have -no-init-file).  So I renamed it and tried again in the cloned ESS3.
> Results attached.  Lots of warnings, but again it seems to get through
> the ess-sas stuff that was a problem, only to fail with
> ess-r-xref.el:32:1:Error: Cannot open load file: no such file or
> directory, xref
>
Apparently xref is used in emacs 25; I'm running emacs 24.

Ross