Release differences from 1.3:
* Fixed a case of division by zero, most notably when asteroids were
destroyed.
* Added Y as a fire button (for QWERTZ layout users).
* Changed 3-way and 4-way spread to the original behavior.
* Added fake difficulty to the Dorim mission.
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.
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.