(makefile-dependency-regex): Turn it into a var, and refine it to mask one more level...

(makefile-dependency-regex): Turn it into a var, and refine it to mask one more level of nested vars.
(makefile-rule-action-regex): Turn it into a var, and refine it so it recognizes backslashed continuation lines as belonging to the same command.
(makefile-macroassign-regex): Refine it so it recognizes backslashed continuation lines as belonging to the same command.
(makefile-var-use-regex): Don't look at the next char, because it might be the same one to be skipped by the initial [^$], leading to an overlooked variable use.
(makefile-make-font-lock-keywords): Remove two parameters, which are now variables that some of the modes set locally.  Handle dependency and rule action matching through functions, because regexps alone match too often.  Dependency matching now comes last, so it can check, whether a colon already matched something else.
(makefile-mode): Inform that font-lock improves makefile parsing capabilities.
(makefile-match-dependency, makefile-match-action): New functions.