Add Hurd to OS list and disable dv1394 in the Hurd case.

patch by Andres Mejia, mcitadel gmail com

Originally committed as revision 18938 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1697,6 +1697,9 @@ case $target_os in
         ;;
     gnu/kfreebsd)
         ;;
+    gnu)
+        disable dv1394
+        ;;
 
     *)
         die "Unknown OS '$target_os'."