Made the cmake test for C++11 support try to compile a C++11 project if

we don't know what compiler we are dealing with.  If the test compile works
then we assume we have C++11.

dlib/dnn/test_for_cpp11/CMakeLists.txt

+
+cmake_minimum_required(VERSION 2.8.4)
+project(cpp11_test)
+
+add_library(cpp11_test STATIC cpp11_test.cpp )

dlib/dnn/test_for_cpp11/cpp11_test.cpp

+// Copyright (C) 2015  Davis E. King (davis@dlib.net)
+// License: Boost Software License   See LICENSE.txt for the full license.
+
+#include <memory>
+#include <iostream>
+
+using namespace std;
+
+class testme
+{
+public:
+
+    testme(testme&&) = default;
+    testme(const testme&) = delete;
+
+    void dostuff()
+    {
+        auto x = 4;
+
+        decltype(x) asdf = 9;
+
+        auto f = [](){ cout << "in a lambda!" << endl; };
+        f();
+    }
+
+    template <typename ...T>
+    void variadic_template(
+        T&& ...args
+    )
+    {
+    }
+
+    std::shared_ptr<int> asdf;
+};
+
+// ------------------------------------------------------------------------------------
+

dlib/use_cpp_11.cmake

@@ -5,10 +5,17 @@
 cmake_minimum_required(VERSION 2.8.4)
 
 # Don't rerun this script if its already been executed.
-if (COMPILER_CAN_DO_CPP_11)
+if (DEFINED COMPILER_CAN_DO_CPP_11)
    return()
 endif()
 
+if (POLICY CMP0054)
+    cmake_policy(SET CMP0054 NEW)
+endif()
+
+# Set to false unless we find out otherwise in the code below.
+set(COMPILER_CAN_DO_CPP_11 0)
+
 # Determine the path to dlib.
 string(REGEX REPLACE "use_cpp_11.cmake$" "" dlib_path ${CMAKE_CURRENT_LIST_FILE})
 include(${dlib_path}/add_global_compiler_switch.cmake)
@@ -33,6 +40,17 @@ if (CMAKE_VERSION VERSION_LESS "3.1")
          add_global_compiler_switch("-std=c++11")
          set(COMPILER_CAN_DO_CPP_11 1)
       endif()
+   else()
+      # Since we don't know what compiler this is ust try to build a c++11 project and see if it compiles.
+      message(STATUS "Building a C++11 test project to see if your compiler supports C++11")
+      try_compile(test_for_cpp11_worked ${PROJECT_BINARY_DIR}/cpp11_test_build 
+         ${dlib_path}/dnn/test_for_cpp11 cpp11_test)
+      if (test_for_cpp11_worked)
+         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.***")
+      endif()
    endif()
 else()
    # Set a flag if the compiler you are using is capable of providing C++11 features.