add check for pcap.h in configure.ac
add check for pcap.h in configure.ac

daodaoliang/tcpcopy,ycaihua/tcpcopy,horryq/tcpcopy,liviusATnetskope/second,caosiyang/tcpcopy,ycaihua/tcpcopy,wfxiang08/tcpcopy,u20024804/tcpcopy,wfxiang08/tcpcopy,u20024804/tcpcopy,sshling/tcpcopy,left2right/tcpcopy,eliteYang/tcpcopy,eliteYang/tcpcopy,horryq/tcpcopy,daodaoliang/tcpcopy,left2right/tcpcopy,liviusATnetskope/second,sshling/tcpcopy,caosiyang/tcpcopy