libtorrent: update to 0.13.6, disable ipv6, remove ipv6 hack

Fix libtorrent compilation by:
* Disable ipv6
* Remove the problematic patches/120-... ipv6 hack at the same time,
  as it is a 5-year old hack to fix one ipv6 problem.
* Update libtorrent to git master HEAD (0.13.6).

Libtorrent compilation has been broken due to patches/120-...
since #1181 got merged and ipv6 got enabled.
Users have seen issues like #1316 and #1804

IPv6 support in libtorrent master is not compelete.
Instead there is a separate ipv6 branch, which still needs some
cleanup before mainstream use. See discussion at
https://github.com/rakshasa/rtorrent/issues/59#issuecomment-56651538
So, it makes no sense to use ipv6 with the master branch.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

libs/libtorrent/Makefile

@@ -1,5 +1,5 @@
 #
-# Copyright (C) 2007-2013 OpenWrt.org
+# Copyright (C) 2007-2015 OpenWrt.org
 #
 # This is free software, licensed under the GNU General Public License v2.
 # See /LICENSE for more information.
@@ -8,13 +8,13 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libtorrent
-PKG_VERSION:=0.13.4-git-1
+PKG_VERSION:=0.13.6-git-1
 PKG_RELEASE=$(PKG_SOURCE_VERSION)
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://github.com/rakshasa/libtorrent.git
 PKG_SOURCE_SUBDIR:=$(PKG_NAME)-$(PKG_VERSION)
-PKG_SOURCE_VERSION:=72e908707f01ee01a9b4918436c64348878b63f7
+PKG_SOURCE_VERSION:=14e793b75dac95c51ad64ff9cd2dc6772b68c625
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION)-$(PKG_SOURCE_VERSION).tar.gz
 
 PKG_FIXUP:=autoreconf
@@ -50,7 +50,7 @@ CONFIGURE_ARGS+= \
 	--enable-openssl \
 	--disable-instrumentation \
 	--with-zlib=$(STAGING_DIR)/usr \
-	$(call autoconf_bool,CONFIG_IPV6,ipv6)
+	--disable-ipv6
 
 define Build/InstallDev
 	$(INSTALL_DIR) $(1)/usr/include