json.o: In function `ast_json_vstringf':
/usr/src/asterisk-13.38.1/main/json.c:487: undefined reference to `json_vsprintf'
/usr/src/asterisk-13.38.1/main/json.c:487: undefined reference to `json_vsprintf'
collect2: error: ld returned 1 exit status
make[1]: *** [asterisk] Error 1
make: *** [main] Error 2
这是什么原因?