Improved error messages about non-C++11 compilers and made the example cmake

file a little clearer on this issue.

dlib/cmake_utils/use_cpp_11.cmake

@@ -47,7 +47,7 @@ if (CMAKE_VERSION VERSION_LESS "3.1.2")
          message(STATUS "C++11 activated.")
          set(COMPILER_CAN_DO_CPP_11 1)
       else()
-         message(STATUS "*** Your compiler failed to build a C++11 project, so dlib won't use C++11 features.***")
+         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 CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0.24215.1 ) 
@@ -55,6 +55,9 @@ elseif(MSVC AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0.24215.1 )
    message(STATUS "So we aren't enabling things that require full C++11 support (e.g. the deep learning tools).")
    message(STATUS "Also, be aware that Visual Studio's version naming is confusing, in particular, there are multiple versions of 'update 3'")
    message(STATUS "So if you are getting this message you need to update to the newer version of Visual Studio to use full C++11.")
+   set(USING_OLD_VISUAL_STUDIO_COMPILER 1)
+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.")
 else()  
 
    # Set a flag if the compiler you are using is capable of providing C++11 features.
@@ -79,7 +82,8 @@ else()
    endif()
 endif()
 
-# Always enable whatever partial C++11 support we have, even if it isn't full support.
+# Always enable whatever partial C++11 support we have, even if it isn't full
+# support, and just hope for the best.
 if (NOT COMPILER_CAN_DO_CPP_11)
    include(CheckCXXCompilerFlag)
    CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)

examples/CMakeLists.txt

@@ -38,8 +38,11 @@ MACRO(add_gui_example name)
    endif()
 ENDMACRO()
 
-# The deep learning toolkit requires a C++11 capable compiler.  
-if (COMPILER_CAN_DO_CPP_11)
+# The deep learning toolkit requires a compiler with essentially complete C++11
+# support.  However, versions of Visual Studio prior to October 2016 didn't
+# provide enough C++11 support to compile the DNN tooling, but were good enough
+# to compile the rest of dlib.  
+if (NOT USING_OLD_VISUAL_STUDIO_COMPILER)
    add_example(dnn_introduction_ex)
    add_example(dnn_introduction2_ex)
    add_example(dnn_inception_ex)