(font-lock-compile-keywords): New arg `regexp'.

(font-lock-compile-keywords): New arg `regexp'.
If set and if applicable, add a regexp to highlight defun-like
text inside comments and strings.
(font-lock-fontify-keywords-region): Pass that new arg.
(font-lock-set-defaults): Move the code to set `font-lock-keywords'
to the end and pass that new arg.
(c-font-lock-keywords-2): Fix regex for labels.
(font-lock-match-c++-style-declaration-item-and-skip-to-next):
Make it work when LIMIT is several lines further.
(c-font-lock-keywords-3, c++-font-lock-keywords-3)
(objc-font-lock-keywords-3, java-font-lock-keywords-3):
Use backquote and make the regexes for `int a, b, c;' work on
multiple lines.