Updated C++11 enabling script to work around some kind of bug in cmake related

to new versions of clang on OSX, without this extra statement C++11 wasn't
being enabled.

dlib/cmake_utils/use_cpp_11.cmake

@@ -79,6 +79,9 @@ else()
       if (NOT CMAKE_CXX_STANDARD OR CMAKE_CXX_STANDARD LESS 11)
          set(CMAKE_CXX_STANDARD 11)
          set(CMAKE_CXX_STANDARD_REQUIRED YES)
+         if (APPLE AND "${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+            add_global_compiler_switch("-std=c++11")
+         endif()
          message(STATUS "C++11 activated.")
       endif()
    endif()