It seems that you don't have WARN defined, this is probably some debug option. It should probably defined as
#define WARN Serial.println
But since you are probably using Serial for connection with the ESP, you have to disable it (don't ask me how, I've never used AT).
pert:
Be aware that it outputs debug information on Serial by default so you probably need to disable that if your ESP8266 is connected to Serial.
I would like to point out that the ESP8266 Arduino core is in most (if not all) aspects superior to the AT firmware. AT + Arduino is convoluted, firmware version sensitive, inefficient, extremely slow ...
Also, by doing all WiFi and HTTP related stuff on the ESP itself, you don't have to waste resources on it on the Arduino side, and your Arduino code will be many times simpler, since you don't have to focus on the networking stuff, just send the value over Serial, without having to worry about how your value will arrive at its destination, you can just trust the ESP to handle all that for you.