pfring: workaround potential librt deps

It seems some version of pfring needs to be link with librt.

configure.ac

@@ -876,12 +876,18 @@
         LIBPFRING=""
         AC_CHECK_LIB(pfring, pfring_open,, LIBPFRING="no", [-lpcap])
         if test "$LIBPFRING" = "no"; then
-            if test "x$enable_pfring" = "xyes"; then
+            LIBPFRING=""
-            echo
+            AC_CHECK_LIB(pfring, pfring_stats,, LIBPFRING="no", [-lpcap -lrt])
-            echo "   ERROR! --enable-pfring was passed but the library was not found or version is >4, go get it"
+            if test "$LIBPFRING" = "no"; then
-            echo "   from http://www.ntop.org/PF_RING.html"
+                if test "x$enable_pfring" = "xyes"; then
-            echo
+                echo
-            exit 1
+                echo "   ERROR! --enable-pfring was passed but the library was not found or version is >4, go get it"
+                echo "   from http://www.ntop.org/PF_RING.html"
+                echo
+                exit 1
+                fi
+            else
+                LIBS="${LIBS} -lrt"
             fi
         fi