build: resolve link failure for ip6t_NETMAP

build: resolve link failure for ip6t_NETMAP

Link stage of libip6t_NETMAP failed since recently.

  CCLD     libip6t_NETMAP.so
/usr/lib64/gcc/x86_64-suse-linux/4.7/../../../../x86_64-suse-linux/bin/ld:
cannot find -lip6tc

libip6t_NETMAP.c uses the "ipv6_prefix_length" function from
libip6tc.so; "-lip6tc" is used in the Makefile, but, the directory to
it is not specified.

Why does the link succeed for some people? Because
/usr/lib(64)/libip6tc.so satisfies -lip6tc, but not all environments,
especially those without iptables development files, have that file,
hence this link error can happen.

By suggestion of Mike Frysinger, this patch uses libtool to produce
and link the plugins.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>