handle backslash in raw string in Emacs 23

Go-mode in Emacs 23 does not recognize a backslash followed
by a backquote as end of raw string literal, as it does not
support syntax-propertize-function which Go-mode uses to
remove special meaning from backslashes in ``.

This patch provides a fallback mechanism to do the same thing
using font-lock-syntactic-keywords, which is supported by
Emacs 23.
do not highlight built-in function if not followed by '('

Name of built-in function is not reserved word in Go, and you can
use it as variable name. "new" is often used as local variable, for
instance.

This patch is to apply font-lock-builtin-face only when built-in
function name is followed by '(', so that it doesn't highlight
non-function variable that happen to have the same name as built-in
function.