(makefile-dependency-skip): New variable.

(makefile-dependency-skip): New variable.
(makefile-previous-dependency): Inline the new matcher, because it is too complex to work in both directions.
(makefile-match-dependency): Eliminate `backward' arg (see above). Completely reimplemented so as to not sometimes go into an endless loop.  It should also be more efficient, because first it only searches for `:', instead of applying the very complex regexp.