[SDL] Adding a new video driver to the build

[Brian Gunn]

How can I add a new video driver to the build?  I managed to get it to "work"
somewhat by modifying the Makefile, but it still doesn't work like I want
because I still need to link the test programs with my SDL_XXX_video.o.  And I'm
sure modifying the Makefile isn't the right way to do this anyway since it's
created from Makefile.in.

What's the correct way to add a new video driver to the build?

Thank you!

Brian