Obsolete SDL headers in emscripten/incoming/system/include/SDL

[jyaif]

After running the instructions to install Binaryen/Emscripten by source:

$ git clone https://github.com/juj/emsdk.git
$ cd emsdk
$ ./emsdk install sdk-incoming-64bit binaryen-master-64bit
$ ./emsdk activate sdk-incoming-64bit binaryen-master-64bit

emsdk/emscripten/incoming/system/include/SDL/SDL_version.h contains:

#define SDL_MAJOR_VERSION   1
#define SDL_MINOR_VERSION   3
#define SDL_PATCHLEVEL      0

whereas https://github.com/emscripten-ports/SDL2/blob/master/include/SDL_version.h contains:

#define SDL_MAJOR_VERSION   2
#define SDL_MINOR_VERSION   0
#define SDL_PATCHLEVEL      5

[kripken]

The SDL directory contains SDL1.3, and the SDL2 port in ports contains SDL2.0. Some projects still use 1.3, so we haven't removed support for it.