sdl: Add optional support for X11 video driver

The Quartz driver in the SDL 1.2 library doesn't work with wrappers
(like go, haskell, ...) which hijack the main() function (as opposed to
using SDL_main). One solution is to use the (deprecated) X11 SDL video
driver backend, in combination with the open source XQuartz server.
Homebrew's current formula builds SDL without the X11 backend by
default. This change adds an optional installation option that
re-enables the X11 backend.