Link with pthread on Linux

CMakeLists.txt

@@ -131,6 +131,10 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
     endif()
 endif()
 
+if(NOT APPLE AND NOT MSVC)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
+endif()
+
 #
 # Declare source and header files.
 #