STEP 1: Copy driver and include folders to your project sub-folder, such as app folder. Unused drivers can be removed in your project.
STEP 2: Modify Makefile in app folder.
1). Search SUBDIRS, add driver as subdir:
SUBDIRS= \
user \
driver
2). Search COMPONENTS_eagle.app.v6, add libdriver.a:
COMPONENTS_eagle.app.v6 = \
user/libuser.a \
driver/libdriver.a