SDL2 upstreams OS/2 Support

If 2020 couldn't get more peculiar, today the SDL2 project mainlined support for the OS/2 operating system.

While OS/2 is no longer maintained by IBM and was never really a gaming platform for where SDL2 is most commonly used, this software library that serves as an abstraction layer for multimedia/gaming hardware components and software platforms has merged the OS/2 port.

Neat.