Revert "build: resolve link failure for ip6t_NETMAP"

Revert "build: resolve link failure for ip6t_NETMAP"

This reverts commit 68e77a26111ee6b8f10c735a76891a7de6d57ee6.

The use of libtool was introduced to resolve linking problems
in NETMAP (IPv6 version), but that resulted in RPATH problems
reported from distributors and warnings spotted by libtool at
linking stage.

Since (0ca548b libip6t_NETMAP: Use xtables_ip6mask_to_cidr and
get rid of libip6tc dependency) fixed the NETMAP issue, let's
roll back to our previous stage.

A small conflicts in extensions/GNUmakefile.in has been resolved
in this revert.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>