acceleration-Grbl now works with atmega 168 by disabling arc motion

motion_control.c

@@ -42,6 +42,7 @@ void mc_dwell(uint32_t milliseconds)
 // axis in axis_l which will be the axis for linear travel if you are tracing a helical motion.
 // position is a pointer to a vector representing the current position in millimeters.
 
+#ifdef __AVR_ATmega328P__
 // The arc is approximated by generating a huge number of tiny, linear segments. The length of each 
 // segment is configured in settings.mm_per_arc_segment.  
 void mc_arc(double theta, double angular_travel, double radius, double linear_travel, int axis_1, int axis_2, 
@@ -77,6 +78,7 @@ void mc_arc(double theta, double angular_travel, double radius, double linear_tr
   }
   plan_set_acceleration_manager_enabled(acceleration_manager_was_enabled);
 }
+#endif
 
 void mc_go_home()
 {