Toggle whitespace matching mode with M-s SPC.

Toggle whitespace matching mode with M-s SPC.
http://lists.gnu.org/archive/html/emacs-devel/2012-09/msg00008.html

* lisp/isearch.el (search-whitespace-regexp): Doc fix.
Remove cons cell customization.
(isearch-mode-map): Bind "\M-s " to isearch-toggle-lax-whitespace.
(isearch-lax-whitespace, isearch-regexp-lax-whitespace):
New variables.
(isearch-forward, isearch-forward-regexp): Doc fix.
(isearch-toggle-lax-whitespace): New command.
(search-forward-lax-whitespace, search-backward-lax-whitespace)
(re-search-forward-lax-whitespace)
(re-search-backward-lax-whitespace): New functions.
(isearch-whitespace-regexp): Remove function.
(isearch-query-replace): Let-bind replace-search-function and
replace-re-search-function.
(isearch-occur): Let-bind search-spaces-regexp according to the
value of isearch-lax-whitespace and isearch-regexp-lax-whitespace.
(isearch-quote-char): Check isearch-regexp-lax-whitespace in the
condition for C-q SPC.
(isearch-search-fun-default): Use new functions mentioned above.
(isearch-search-forward, isearch-search-backward): Remove functions.
(isearch-search): Don't let-bind search-spaces-regexp.
(isearch-lazy-highlight-space-regexp): Remove variable.
(isearch-lazy-highlight-lax-whitespace)
(isearch-lazy-highlight-regexp-lax-whitespace): New variables.
(isearch-lazy-highlight-new-loop): Use them.
(isearch-lazy-highlight-search): Don't let-bind search-spaces-regexp.