Made this script work with clang

dlib/use_cpp_11.cmake

@@ -29,6 +29,14 @@ if (CMAKE_VERSION VERSION_LESS "3.1")
          add_global_compiler_switch("-std=gnu++0x")
          set(COMPILER_CAN_DO_CPP_11 1)
       endif()
+   elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+      execute_process( COMMAND ${CMAKE_CXX_COMPILER} --version OUTPUT_VARIABLE clang_full_version_string )
+      string (REGEX REPLACE ".*clang version ([0-9]+\\.[0-9]+).*" "\\1" CLANG_VERSION ${clang_full_version_string})
+      if (CLANG_VERSION VERSION_GREATER 3.1)
+         message(STATUS "C++11 activated.")
+         add_global_compiler_switch("-std=c++11")
+         set(COMPILER_CAN_DO_CPP_11 1)
+      endif()
    endif()
 else()
    # Set a flag if the compiler you are using is capable of providing C++11 features.