Require the compiler to support the thread_local C++11 feature.

dlib/use_cpp_11.cmake

@@ -59,7 +59,9 @@ else()
        ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_lambdas;" AND
        ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_defaulted_move_initializers;" AND
        ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_delegating_constructors;" AND
+       ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_thread_local;" AND
        ";${CMAKE_CXX_COMPILE_FEATURES};" MATCHES ";cxx_auto_type;")
+
       set(COMPILER_CAN_DO_CPP_11 1)
       # Set which standard to use unless someone has already set it to something
       # newer.