Improve Debian/Ubuntu code porting

[RICCIARDI-Adrien]

Hi,

Debian-based distributions install SDL_ttf.h header in /usr/include/SDL2 directory, which would be ~/.emscripten_cache/asmjs/ports-builds/sdl2/include/SDL2/ on emscripten port. However, emscripten port SDL_ttf.h is currently located in ~/.emscripten_cache/asmjs/ports-builds/sdl2/include.
It would be great to automatically copy SDL_ttf.h (or to create a symlink on UNIX) to ~/.emscripten_cache/asmjs/ports-builds/sdl2/include/SDL2/ to avoid an emscripten-specific #ifdef in the program source code when including the file.
On Debian the SDL_ttf.h file is included this way : #include <SDL2/SDL_ttf.h>
With emscripten the include must be modified in : #include <SDL_ttf.h>

Keep it up !
Adrien