原因：缺少librt.so 编译的时候，加入-lrt即可。
LDFLAGS_C = $(LDFLAGS) -shared -Wl,-init,$(MQTTCLIENT_INIT) -lpthread -lrt LDFLAGS_CS = $(LDFLAGS) -shared $(START_GROUP) -lpthread -lrt $(EXTRA_LIB) -lssl -lcrypto $(END_GROUP) -Wl,-init,$(MQTTCLIENT_INIT) LDFLAGS_A = $(LDFLAGS) -shared -Wl,-init,$(MQTTASYNC_INIT) -lpthread -lrt LDFLAGS_AS = $(LDFLAGS) -shared $(START_GROUP) -lpthread -lrt $(EXTRA_LIB) -lssl -lcrypto $(END_GROUP) -Wl,-init,$(MQTTASYNC_INIT)
参考文章：
https://bbs.csdn.net/topics/390654806
https://www.baidu.com/link?url=PDw02u49rFDTAPdPUO2EWarWs2pN0en4PJs6Tg9BLOaJQ4mCeDO-X2_cdJpSXfWB&wd=&eqid=f5bd165d0012619c000000045dbd0127
