use of flet in notmuch-emacs

[David Bremner]

Today I noticed

,----
| emacs --quick --directory emacs -batch -f batch-byte-compile emacs/notmuch-show.el
| 
| In notmuch-show-view-part:
| emacs/notmuch-show.el:541:22:Warning: `flet' is an obsolete macro (as of
|     24.3); use either `cl-flet' or `cl-letf'.
`----

Chatter on #emacs suggests that cl-flet is not in fact a replacement for
our use case since it does not do dynamic binding. I'm not really 100%
sure what the best plan is, but the old definition of flet is available
at

        https://github.com/sigma/el-x/blob/master/lisp/dflet.el

d