Not able to compile

[sunilsud]

libraries/aws-mqtt-websockets-master/AWSWebSocketClient.cpp.o: In function AWSWebSocketClient::ntpFixNumber(int)': /Users/sunil/Documents/Arduino/libraries/aws-mqtt-websockets-master/AWSWebSocketClient.cpp:526: undefined reference to gmtime_r'
libraries/aws-mqtt-websockets-master/AWSWebSocketClient.cpp.o:(.text._ZN18AWSWebSocketClient17getCurrentTimeNTPEv+0x33): undefined reference to gmtime_r' libraries/aws-mqtt-websockets-master/AWSWebSocketClient.cpp.o: In function AWSWebSocketClient::getCurrentTimeNTP()':
/Users/sunil/Documents/Arduino/libraries/aws-mqtt-websockets-master/AWSWebSocketClient.cpp:526: undefined reference to `gmtime_r'
collect2: error: ld returned 1 exit status
exit status 1