undefined reference to `openpty' error

Recently ,I need to compile pptp-linux by myself.But there was an error:

gcc -o pptp pptp.o pptp_gre.o ppp_fcs.o pptp_ctrl.o dirutil.o vector.o inststr.o util.o version.o test.o pptp_quirks.o orckit_quirks.o pqueue.o pptp_callmgr.o routing.o pptp_compat.o  
pptp.o: In function `main':
/home/agile/oss/pptp-linux/pptp.c:327: undefined reference to `openpty'

It means I miss something
Actually add the -lutil flag ,it can compile ok.