bugfix for boards with a timer 4 but no channel D

compilation failed for Mega because the COM4D1 and OCR4D registers are defined for 32U4 but not for Mega

Author: zeveland

hardware/arduino/cores/arduino/wiring_analog.c

@@ -230,7 +230,7 @@ void analogWrite(uint8_t pin, int val)
 				break;
 			#endif
 
-			#if defined(TCCR4C)
+			#if defined(TCCR4C) && defined(COM4D1)
 			case TIMER4D:				
 				// connect pwm to pin on timer 4, channel D
 				sbi(TCCR4C, COM4D1);