po-mode.el from gettext-0.11.1 does not work with emacs20

[Santiago Vila]

Hello.

I have installed po-mode.el from gettext-0.11.1, and now, in emacs20,
when I try to edit some .po or .pot file I get:

File mode specification error: (void-function mapc)

and po-mode is not loaded. If I try to load po-mode by hand doing
"M-x po-mode" I get:

Symbol's function definition is void: mapc

In emacs21, however, it works.

I'm using emacs20_20.7-10.1 and emacs21_21.1-7 from Debian woody.

My emacs startup file is as follows:

(setq auto-mode-alist
(cons '("\\.po[tx]?\\'\\|\\.po\\." . po-mode) auto-mode-alist))
(autoload 'po-mode "gettext/po-mode"
  "Major mode for translators to edit PO files" t)

[ Note: "gettext/po-mode" is the appropriate thing for po-mode when it
  comes from the Debian package ].