Port current-time change to XEmacs 21.4.

See the buildbot log at:
http://www.randomsample.de:4456/builders/xemacs21.4-linux/builds/1285
* lisp/erc/erc.el (erc-emacs-time-to-erc-time)
(erc-emacs-time-to-erc-time):
* lisp/gnus/gnus-util.el (gnus-float-time):
* lisp/org/org-compat.el (org-float-time):
Use 2-arg defalias, since XEmacs 21.4 doesn't support 3-arg.