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

motion_control.h

@@ -31,12 +31,14 @@
 // NOTE: Although this function structurally belongs in this module, there is nothing to do but
 // to forward the request to the planner. For efficiency the function is implemented with a macro.
 
+#ifdef __AVR_ATmega328P__
 // Execute an arc. theta == start angle, angular_travel == number of radians to go along the arc,
 // positive angular_travel means clockwise, negative means counterclockwise. Radius == the radius of the
 // circle in millimeters. axis_1 and axis_2 selects the circle plane in tool space. Stick the remaining
 // axis in axis_l which will be the axis for linear travel if you are tracing a helical motion.
 void mc_arc(double theta, double angular_travel, double radius, double linear_travel, int axis_1, int axis_2, 
   int axis_linear, double feed_rate, int invert_feed_rate, double *position);
+#endif
   
 // Dwell for a couple of time units
 void mc_dwell(uint32_t milliseconds);