libmodbus: update to 3.1.2

Although this version is marked 'unstable' by upstream, it's very
robust and stable. So give it a broader audience for testing.

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

libs/libmodbus/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.
@@ -8,12 +8,12 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libmodbus
-PKG_VERSION:=3.0.6
+PKG_VERSION:=3.1.2
 PKG_RELEASE:=1
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=http://libmodbus.org/releases
-PKG_MD5SUM:=c80f88b6ca19cabc4ceffc195ca07771
+PKG_MD5SUM:=dd4262f87ed835c4d1e560f8321a222c
 
 PKG_MAINTAINER:=Michael Heimpold <mhei@heimpold.de>
 
@@ -36,6 +36,7 @@ define Package/libmodbus/description
   A Modbus library for Linux, Mac OS X, FreeBSD, QNX and Win32.
 endef
 
+CONFIGURE_ARGS += --without-documentation
 TARGET_CFLAGS += $(FPIC)
 
 define Build/InstallDev