(makefile-imenu-generic-expression): Use function to find dependencies, because regex...

(makefile-imenu-generic-expression): Use function to find dependencies, because regexp alone is so complex, it easily goes into an endless loop.
(makefile-makepp-mode): Also add submenu for Perl functions defined in the makefile.
(makefile-bsdmake-mode): Special imenu-generic-expression no longer needed, due to function call.
(makefile-match-dependency): Take BOUND into account when checking if we're through.