emacs: jump: fix compile warning on emacs 23

emacs: jump: fix compile warning on emacs 23

notmuch-jump uses window-body-width which is not defined in emacs
23. To get around this it does

(unless (fboundp 'window-body-width)
  ;; Compatibility for Emacs pre-24
  (defalias 'window-body-width 'window-width))

This makes sure window-body-width is defined and all should be
well. But it seems that the byte compiler does not realise that this
guarantees that window-body-width will be defined and so, when
compiling with emacs 23, it gives an error

In end of data:
notmuch-jump.el:172:1:Warning: the function `window-body-width' is not known to be defined.

Domo and I came to following on irc: wrap the (unless (fboundp ...))
inside eval-and-compile which ensures that both the test and the
defalias (if needed) happen at both compile and load time.  This fixes
the warning.