compilation error fix: pin for LCD brightness

Firmware/Marlin_main.cpp

@@ -991,9 +991,9 @@ void setup()
 	
 	ultralcd_init();
 
-#if (LCD_BL_PIN != -1)
+#if (LCD_BL_PIN != -1) && defined (LCD_BL_PIN)
 	analogWrite(LCD_BL_PIN, 255); //set full brightnes
-#endif //(LCD_BL_PIN != -1)
+#endif //(LCD_BL_PIN != -1) && defined (LCD_BL_PIN)
 
 	spi_init();