Add '-lpthread' option for some Linux systems

According to [FindBoost.cmake](https://github.com/Kitware/CMake/blob/v3.0.2/Modules/FindBoost.cmake), cmake doesn't generate '-lpthread' option for Boost_LIBRARIES. It seems that only debian (and ubuntu) system has a specialized patch for this issue as we've discussed. So i'm adding the missing DSO info there for other linux systems.

Tested on Gentoo Linux with cmake 3.0.2. Still needs to be tested on other systems.