Use SDL instead of sdl for extra-libraries

The problem I had with my machine was that SDL2-image wouldn't compile because of the "extra-libraries: sdl2" line in the cabal file. I have /usr/lib64/libSDL2.so (upper case), so cabal couldn't find the .so file. Are you working on a Windows machine and didn't notice because of that? Or does your distribution ship with libsdl2.so?