Code:
void LCD_ausgabe()
{
    static int last_Celsius   = -1000;
    static int last_soll_temp = -1000;

    if (last_soll_temp != soll_temp)
      {
         last_soll_temp = soll_temp;

         lcd.setCursor(0, 0);
         lcd.print("Soll Temp:    C");
         lcd.setCursor(11, 0);
         lcd.print(soll_temp);

      }
    if (last_celsius != celsius)
      {
         last_celsius = celsius;

         lcd.setCursor(0, 1);
         lcd.print("Ist Temp:    C");
         lcd.setCursor(10, 1);
         lcd.print(celsius);
      }
}