Fix enable/disable of libnet

--- a/configure.ac	2010-12-06 18:17:03.000000000 +0100
+++ b/configure.ac	2011-01-11 13:22:16.271977100 +0100
@@ -928,7 +928,7 @@
 
 AC_ARG_ENABLE([libnet],
  [  --enable-libnet 	Use libnet for ARP based funcationality, [default=try]], 
- [enable_libnet=$withval], [enable_libnet=try])
+ [], [enable_libnet=try])
 
 libnet=""
 libnet_version="none"