autotools: add libpcap dependencyto pfring for checks.

PF_RING seems to depend on pcap if bfp filter is activated. For this
reason, not having the dependency during configure test causes a
failure in feature detection.

configure.in

@@ -657,7 +657,7 @@ esac
         fi
 
         LIBPFRING=""
-        AC_CHECK_LIB(pfring, pfring_open,, LIBPFRING="no")
+        AC_CHECK_LIB(pfring, pfring_open,, LIBPFRING="no", [-lpcap])
         if test "$LIBPFRING" = "no"; then
             if test "x$enable_pfring" = "xyes"; then
             echo
@@ -669,21 +669,21 @@ esac
         fi
 
         LIBPFRING_ENABLE_RING=""
-        AC_CHECK_LIB(pfring, pfring_enable_ring,, LIBPFRING_ENABLE_RING="no")
+        AC_CHECK_LIB(pfring, pfring_enable_ring,, LIBPFRING_ENABLE_RING="no", [-lpcap])
         if test "$LIBPFRING_ENABLE_RING" != "no"; then
             AC_DEFINE([HAVE_PFRING_ENABLE],[1],[PF_RING pfring_enable_ring is available])
         fi
 
         LIBPFRING_CLUSTER_TYPE=""
         AC_CHECK_LIB(pfring, pfring_set_cluster,
-                        , LIBPFRING_CLUSTER_TYPE="no")
+                        , LIBPFRING_CLUSTER_TYPE="no", [-lpcap])
         if test "$LIBPFRING_CLUSTER_TYPE" != "no"; then
             AC_DEFINE([HAVE_PFRING_CLUSTER_TYPE],[1],[PF_RING pfring_set_cluster is available])
         fi
 
         LIBPFRING_BPF_FILTER=""
         AC_CHECK_LIB(pfring, pfring_set_bpf_filter,
-                        , LIBPFRING_BPF_FILTER="no")
+                        , LIBPFRING_BPF_FILTER="no", [-lpcap])
         if test "$LIBPFRING_BPF_FILTER" != "no"; then
             AC_DEFINE([HAVE_PFRING_SET_BPF_FILTER],[1],[PF_RING pfring_set_bpf_filter is available])
         fi