Fix creating of $(OBJDIR) in a proper way

This adds a dependency on $(OBJDIR) for every rule that creates a file
inside $(OBJDIR) but does not already depend on any file within
$(OBJDIR). All other dependencies on $(OBJDIR) are removed.

These dependencies are added after a | to tell make that this is a
"order-only prerequisite". This means that the file inside $(OBJDIR)
needs $(OBJDIR) to be present but if $(OBJDIR) changes (which happens
whenever a file _inside_ the directory is touched!), there is no need to
recompile the file within $(OBJDIR).

Implementing this using a generic implicit rule like:

$(OBJDIR)/%: | $(OBJDIR)

doesn't work, since make doesn't merge the prerequisites of multiple
implicit rules like it does for explicit rules. We could use
$(LOCAL_OBJS) and friends to create explicit rules to do something like
the above, but just adding the dependencies on all rules seems more
explicit and future-proof.

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