Don't assume that $(libdir) is lib. Rely on linker defaults instead.
--- a/configure.ac
+++ b/configure.ac
@@ -55,13 +55,6 @@ AC_ARG_WITH([libnet-prefix],
             [ LIBNET_PREFIX=$withval ],
             [ LIBNET_PREFIX=/usr ])
 	    
-AC_CHECK_FILE([$LIBNET_PREFIX/lib/libnet.a],
-              [LDFLAGS="-L$LIBNET_PREFIX/lib"
-	       CPPFLAGS="-I$LIBNET_PREFIX/include" ],
-              AC_MSG_ERROR([
-Libnet-1.1.1 Packet Shaping Library is required!
-Verify to have installed it and pass --with-libnet-prefix to configure.
-Or download it from http://www.packetfactory.net/libnet ])) 
               
 AC_CHECK_LIB(net, libnet_name2addr4,, 
 AC_MSG_ERROR([