Missing check for libsdl2 dependency on unix build

[StefanoMarina]

In linux mint (debian), trying to launch build.sh will result in

In file included from src/api/renderer.c:2:0:
src/renderer.h:4:10: fatal error: SDL2/SDL.h: Missing file or directory
 #include <SDL2/SDL.h>
          ^~~~~~~~~~~~
compilation terminated.
In file included from src/api/renderer_font.c:2:0:
src/renderer.h:4:10: fatal error: SDL2/SDL.h: Missing file or directory
 #include <SDL2/SDL.h>
          ^~~~~~~~~~~~
compilation terminated.
src/api/system.c:1:10: fatal error: SDL2/SDL.h: Missing file or directory
 #include <SDL2/SDL.h>
          ^~~~~~~~~~~~
compilation terminated.

this is easily solved by adding libsdl2 sudo apt-get install libsdl2-dev , it would be a nice addition however to inform about the dependency on the README.md or automatically install libsdl2 inside the build.sh script.