Merge branch 'jn/makedepend' into next

Merge branch 'jn/makedepend' into next

* jn/makedepend:
  Makefile: always remove .depend directories on 'make clean'
  Makefile: tuck away generated makefile fragments in .depend
  Teach Makefile to check header dependencies
  Makefile: list standalone program object files in PROGRAM_OBJS
  Makefile: lazily compute header dependencies
  Makefile: list generated object files in OBJECTS
  Makefile: disable default implicit rules
  Makefile: rearrange dependency rules
  Makefile: transport.o depends on branch.h now
  Makefile: drop dependency on $(wildcard */*.h)
  Makefile: clean up http-walker.o dependency rules
  Makefile: remove wt-status.h from LIB_H
  Makefile: make sure test helpers are rebuilt when headers change
  Makefile: add missing header file dependencies

Conflicts:
	Makefile