Mine

gcc  -g -O2  -L. -o check_icmp  check_icmp.o  -lgen -lsocket
Undefined                       first referenced
 symbol                             in file
gethostbyname                       check_icmp.o  (symbol belongs to implicit dependency /usr/lib/libnsl.so.1)
inet_addr                           check_icmp.o  (symbol belongs to implicit dependency /usr/lib/libnsl.so.1)
inet_ntoa                           check_icmp.o  (symbol belongs to implicit dependency /usr/lib/libnsl.so.1)
ld: fatal: Symbol referencing errors. No output written to check_icmp
collect2: ld returned 1 exit status
make[2]: *** [check_icmp] Error 1
make[2]: Leaving directory `/export/home/reppocs/nagios-plugins-1.4/plugins'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/export/home/reppocs/nagios-plugins-1.4'
make: *** [all] Error 2