(cl-defstruct): Improve handling of unknown options

(cl-defstruct): Improve handling of unknown options

Until now `cl-defstruct` signaled an error when encountering an
unknown option.  It's easy to code and it does the job, but it
doesn't give good location info in the compiler's output,
and it makes it more painful to use not-yet-supported options.
So just signal a warning instead.

* lisp/emacs-lisp/cl-macs.el (cl-defstruct): Warn about unknown
options, instead of signaling an error.