build: turn vertex-clipping.c into a dependency

Making this into a dependency object not only carries the .c files with it, but
it also brings the include directories as well, which means the users can
simply use the object without guessing the paths.

This should help with moving GL-renderer into a new subdirectory.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>