This is silly. The libraries used have absolutely nothing to do with
"Linux". They're POSIX standard libraries, and will work just as
well on Mac OS X and BSD. The actual code might need to be replaced,
but in any case, this isn't something to limit to "Linux" systems.
Using <SDL/SDL.h>, or in this case <SDL2/SDL.h>, and others like this
is not the proper way to include SDL headers. The proper way is to
pass the -lSDL2 flag to the compiler. The place I put it might be
awkward, but I'm planning to replace this hand-written makefile with
Autoconf/Automake scripts, anyway.