Rename SDL_LIBRARY to SDL_LIBRARIES

[groggone]

This is so that you can actually be able to compile BetterSpades with the 'ENABLE_SDL' flag and not get compilation errors like 'undefined reference to SDL_blahblahblah'

[xtreme8000]

for future reference https://github.com/libsdl-org/SDL/blob/SDL2/SDL2Config.cmake.in