(makefile-targets-face, makefile-shell-face, makefile-makepp-perl-face): New faces.

(makefile-targets-face, makefile-shell-face, makefile-makepp-perl-face): New faces.
(makefile-dependency-regex): Fix it to not make the colon in $(var:a=b) special.
(makefile-rule-action-regex): New regexp for highlighting embedded Shell strings.
(makefile-macroassign-regex): Handle != for highlighting as embedded Shell strings.
(makefile-var-use-regex): New const.
(makefile-statements, makefile-automake-statements)
(makefile-gmake-statements, makefile-makepp-statements)
(makefile-bsdmake-statements): New consts.
(makefile-make-font-lock-keywords): New function.
(makefile-automake-font-lock-keywords)
(makefile-gmake-font-lock-keywords)
(makefile-makepp-font-lock-keywords)
(makefile-bsdmake-font-lock-keywords): New consts.
(makefile-mode-map): Add switchers between the various submodes.
(makefile-mode): Document the availability of the variants.
(makefile-automake-mode, makefile-gmake-mode)
(makefile-makepp-mode, makefile-bsdmake-mode): New derived modes.