find-func broken in 21.4

[Ilya N. Golubev]

Observing only in 21.4.

After your 2006-07-07 `find-func.el' change `find-function' always
issues the following error message.

Symbol's value as variable is void: source-root

This variable exists only in 21.5 since 1 Mar 2003 07:25:27 -0000.

Please test changes on 21.4 before committing.

Anyway, always trying to use such a value even when finding source of
elisp function (even if it is not dumped) is suspicious by itself.
The location of file defining it may be determined only by elisp load
history and generally has nothing to do with location of xemacs
sources, which do not even have to exist in run time.