added missing -pthread compiler flag

CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.6)
 project(cpp11NHF2_chat)
 
 set(CMAKE_CXX_STANDARD 14)
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -pedantic -pthread")
 
 find_package(Boost COMPONENTS system REQUIRED)
 include_directories(${Boost_INCLUDE_DIR})