essentially, add Zero support by shimming in the missing functions

Adafruit_MQTT.cpp

@@ -21,6 +21,14 @@
 // SOFTWARE.
 #include "Adafruit_MQTT.h"
 
+#ifndef dtostrf
+static char *dtostrf (double val, signed char width, unsigned char prec, char *sout) {
+  char fmt[20];
+  sprintf(fmt, "%%%d.%df", width, prec);
+  sprintf(sout, fmt, val);
+  return sout;
+}
+#endif
 
 void printBuffer(uint8_t *buffer, uint8_t len) {
   for (uint8_t i=0; i<len; i++) {

Adafruit_MQTT.h

@@ -22,10 +22,11 @@
 #ifndef _ADAFRUIT_MQTT_H_
 #define _ADAFRUIT_MQTT_H_
 
-#if ARDUINO >= 100
- #include "Arduino.h"
-#else
- #include "WProgram.h"
+#include "Arduino.h"
+
+#ifdef ARDUINO_SAMD_ZERO
+#define strncpy_P(dest, src, len) strncpy((dest), (src), (len))
+#define strncasecmp_P(f1, f2, len) strncasecmp((f1), (f2), (len))
 #endif
 
 // Uncomment/comment to turn on/off debug output messages.
@@ -33,6 +34,7 @@
 
 // Set where debug messages will be printed.
 #define DEBUG_PRINTER Serial
+// If using something like Zero or Due, change the above to SerialUSB
 
 // Define actual debug output functions when necessary.
 #ifdef MQTT_DEBUG