fixed up SDL2 linkage on linux + misc minor changes
- set SDL_STATIC back to ON in SDL2's CMakeLists.txt. building the
  static lib can be disable when running cmake. updated BuildWin32.bat
- link Scream to SDL2 shared lib
- added macro to declare variables as unused.
set SDL_STATIC in Scream's CMakeLists.txt + changed WITH_SDL to a config option
- better way to disable building of the static lib
- updated BuildWin32.bat. probably a bad idea to keep this in the repo.
  encourages hacks that might eventually cause issues.
- also made WITH_SDL an option that can be set when running cmake