Switch by default to SDL2 for Mupen64Plus 2.1

The current version of SDL is 2.0.3. All new releases of distributions contain
this library already. Current Linux distributions and OpenBSD even backporting
patches from Mupen64Plus 2.1 repo to use SDL 2. Android and other mobile
platform don't have support for legacy SDL 1.2.

People like Anthony J. Bentley and Riley Labrecque already requested
a new release of Mupen64Plus 2.1 with SDL2
SDL 1.2 is not deprecated for us, and we shouldnt frighten the user with 6 lines of exclamation points