Rewrite `flet`-usage in terms of `funcall`

Alas, `flet` becomes officially obsolete starting with Emacs 24.3;
Otoh, `cl-flet` is not available in the backported `cl-lib-0.3` (due
to subtle differences between `flet` and `cl-flet`). Thus, to avoid
annoying compile warnings, the easiest course of action is to not use
`flet` altogether.