Split off Makefile into several parts, included from master Makefile

Split off Makefile into several parts, included from master Makefile

* Makefile: just keep help target (now also default for no target)
  and include all other parts from here.
* default.mk: user editable variables with their default values
* maint.mk: definitions by maintainer, should not be touched by user
* targets.mk: user callable targets
* maint-targets.mk: targets for maintenance, should not be called by user
* dependencies.mk: keep order during compile (could become auto-generated)

This is the first in a series of patches that restructures the Makefile to
achieve easier customization and separation of different concerns (mainly user
vs. maintenance of the distribution).  This first patch simply establishes a
set of files without changing anything else to provide a clean starting point.
It uses GNU make extensions since Makefile already depends on GNU make anyway.