change-log-version-number-regexp-list is a variable defined in `add-log.el'.
Its value is

("^(def[^ \n]+[ ]+[^ \n][ ]\"\\([0-9]+.[0-9.]+\\)" "^;+ *Revision: +[^ \n]+[ ]+\\([0-9]+.[0-9.]+\\)")

List of regexps to search for version number.
The version number must be in group 1.
Note: The search is conducted only within 10%, at the beginning of the file.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.