Makefile slight dependency change.
spacemouse_evdev.c: map axis the same way spacenavd/libspnav does by default
spacemouse_evdev.c: fix buffering in that button events do not partially overwrite motion events.
spacemouse_device_read_event: removes buffering return code. Read multiple evdev events until SYN, which means less calls are needed to the function.
libspacemouse.h: update comments.
spacemouse_device_grab/_ungrab: no need to use a variable here.
spacemouse_device_read_event(): return IGNORE when receiving an EV_SYN without a button or motion event buffered.