Fix NAN with real NAN defined in C99

DHT.cpp

@@ -4,8 +4,9 @@ MIT license
 written by Adafruit Industries
 */
 
+#include <math.h>
 #include "DHT.h"
-#define NAN 0
+//#define NAN 0
 
 DHT::DHT(uint8_t pin, uint8_t type, uint8_t count) {
   _pin = pin;