Improved check for old visual studio

dlib/cmake_utils/use_cpp_11.cmake

@@ -18,6 +18,9 @@ set(COMPILER_CAN_DO_CPP_11 0)
 
 include(${CMAKE_CURRENT_LIST_DIR}/add_global_compiler_switch.cmake)
 
+if(MSVC AND MSVC_VERSION VERSION_LESS 1900)
+   message(FATAL_ERROR "C++11 is required to use dlib, but the version of Visual Studio you are using is too old and doesn't support C++11.  You need Visual Studio 2015 or newer. ")
+endif()
 
 # Now turn on the appropriate compiler switch to enable C++11 if you have a
 # C++11 compiler.  In CMake 3.1 there is a simple flag you can set, but earlier
@@ -50,8 +53,6 @@ if (CMAKE_VERSION VERSION_LESS "3.1.2")
          message(FATAL_ERROR "*** Your compiler failed to build a C++11 project, so dlib won't use C++11 features.***")
       endif()
    endif()
-elseif(MSVC AND MSVC_VERSION VERSION_LESS 1900)
-   message(FATAL_ERROR "C++11 is required to use dlib, but the version of Visual Studio you are using is too old and doesn't support C++11.")
 elseif(MSVC AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0.24215.1 ) 
    message(STATUS "NOTE: Visual Studio didn't have good enough C++11 support until Visual Studio 2015 update 3 (v19.0.24215.1)")
    message(STATUS "So we aren't enabling things that require full C++11 support (e.g. the deep learning tools).")