Add a relative path for sdl2 dependencies in setup.py

This pr does not affect the users of 1.9.0, because sdl2 will not be enabled by default.
Its purpose is to make sdl2 testing easier with kivy 1.9.0.

To start testing SDL2 with the current release, add the appropriate libaries to kivy/lib/sdl2 and set the correct environment variables.
A detailed wiki for testers will follow soon.