Rename CreateWindow() to CreateSDLWindow()

CreateWindow() is a symbol internal to Windows. We can't use it in out
code or it will collide.
Correct SDL2 statement for Windows

SDL2 can and should be statically linked. This is much more easier than
linking it dynamically and more comfortable for the user. One libs less
to care about.
Enable SDL2 by default
Use "directsound" as sounddriver when using SDL2 on Windows
Clarify SDL stuff in the README, mention vsync bug with SDL 1.2
Mention that OS X is unsupported

I neither have a Mac nor the time to maintain this platform. SDL 2.0
requires some more complex changes than the usual "two lines to track
Unix and Windows" so suspend OS X support until someone steps up.