As a result, you're not stuck with the weakest weapon in the game
anymore. You have to choose between the double rockets, plasma
permanently upgraded in one area, plasma temporarily upgraded in
some areas, or some combination.
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.