FindBoost: Add system thread library to Boost_LIBRARIES

Add the system thread library to Boost_LIBRARIES when the boost thread
component has been found.

The Boost::thread imported target already pulls in Threads::Threads.
This changes does the same for projects using the Boost_LIBRARIES variable
instead.

Modules/FindBoost.cmake

@@ -2074,6 +2074,9 @@ if(Boost_FOUND)
         message (STATUS "  ${COMPONENT}")
       endif()
       list(APPEND Boost_LIBRARIES ${Boost_${UPPERCOMPONENT}_LIBRARY})
+      if(COMPONENT STREQUAL "thread")
+        list(APPEND Boost_LIBRARIES ${CMAKE_THREAD_LIBS_INIT})
+      endif()
     endif()
   endforeach()
 else()