Merge pull request #30 from igor-bogomolov/master

add os target and optional build soci
cmake_minimum_required (VERSION 2.8.3)
Revert "cmake_minimum_required (VERSION 2.8.3)"

This reverts commit 1546a12fb50fd1257b9bc145e681724322da4849.
add ExternalProject.cmake to support cmake <= 2.8.2