libxml2: fix compilation on archlinux and gentoo

Compilation of libxml2 on some distributions is problematic (at least
archlinux) for OpenWrt. This commit fixes the issue. Issue is caused
because configuration for some reason does not find gzopen from zlib.
This patch issues linker to include zlib anyway, if host system doesn't
have this issue, it is not a problem as linker should not link libs
twice anyway.

Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>

Backported to BB to fix https://dev.openwrt.org/ticket/18295

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

libs/libxml2/Makefile

@@ -1,5 +1,5 @@
 #
-# Copyright (C) 2006-2014 OpenWrt.org
+# Copyright (C) 2006-2015 OpenWrt.org
 #
 # This is free software, licensed under the GNU General Public License v2.
 # See /LICENSE for more information.
@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libxml2
 PKG_VERSION:=2.9.2
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=http://gd.tuwien.ac.at/languages/libxml/ \
@@ -71,7 +71,7 @@ CONFIGURE_ARGS += \
 	--with-xinclude \
 	--with-xpath \
 	--with-xptr \
-	--with-zlib \
+	--with-zlib=$(STAGING_DIR)/usr \
 	--without-lzma
 
 define Build/InstallDev