Including Arduino library in a Makefile for Due
Adding a library isn't as simple as just adding an include path.

You also have to identify each and every source file (or use wildcards as are used for the various sections of the core) and compile them into object files. It is then quite convenient to archive those into a library (.a) file which you can then link your sketch against (if you don't archive you will have to link all the objects files with your sketch in the linking phase).