Use cl-labels instead of cl-flet, safe for emacs 23.

The cl-flet alias doesn't exist in cl-lib-0.3. Making it is a big
mistake. Better use cl-labels with non-lexical-binding behaviour in
emacs-23 (though a warning is printed)

Bug reported by Paul Bowyer <pbowyer@olynet.com>.

* slime-presentations.el: Use cl-labels instead of cl-flet (safe on
emacs 23).

* slime-trace-dialog.el (slime-trace-dialog--open-specs): Use
cl-labels instead of cl-flet (safe on emacs 23).