I think you should do that :
In your .h :
void graphlib_put_sprite_custom(unsigned short x asm("d0"), unsigned short y asm("d1"), void *sprite asm("a0"),void *dest asm ("%a1"));

In a .s file :

 xdef graphlib_put_sprite_custom
graphlib_put_sprite_custom:
  jmp graphlib__0001