PSA: SDL has migrated to GitHub

[spurious]

SDL development(Hg and Bugzilla) is now at GitHub. This repository will not receive any future updates.

• Forum post: https://discourse.libsdl.org/t/sdl-moving-to-github/28700/5
• SDL2: https://github.com/libsdl-org/SDL
• SDL1: https://github.com/libsdl-org/SDL-1.2