A set of new SDL2 related ports has been added, which are essentially
the same as their corresponding port for SDL1.2:
audio/sdl2_mixer
graphics/sdl2_image
graphics/sdl2_ttf
net/sdl2_net
A set of new USE_SDL knobs is available to enable SDL2 and its related
ports to be added as dependency to ports. Use either of
USE_SDL= sdl2 image2 mixer2 net2 ttf2
to pull in the specific sdl2_* or sdl20 port.