[fix / clean / functional]

engine.h:
        seperated definitions needed for engine.c in here, includes prototypes
        changed name from ENGINE_STRUCT to engine_t per advice from IRC

engine.c:
        removed useless header guard
        engine_init returns engine_t now instead of int; doesn't take engine_t as argument anymore
        stopped making audio initialisation return

Makefile:
	wasn't compiling because the quotes in CFLAGS="-some --stuff" was taken literally, fixed
	more warning flags, flag to follow C11 standard

main.c:
	edited to reflect the changes above