CMake tweaks to avoid errors in visual studio

dlib/cmake_utils/set_compiler_specific_options.cmake

 
 cmake_minimum_required(VERSION 2.8.12)
 
+if (POLICY CMP0054)
+    cmake_policy(SET CMP0054 NEW)
+endif()
 
 set(USING_OLD_VISUAL_STUDIO_COMPILER 0)
 if(MSVC AND MSVC_VERSION VERSION_LESS 1900)

dlib/cmake_utils/use_cpp_11.cmake

@@ -80,7 +80,7 @@ else()
             cxx_delegating_constructors
             cxx_thread_local
             cxx_constexpr
-            cxx_decltype_incomplete_return_types
+            # cxx_decltype_incomplete_return_types  # purposfully commented out because cmake errors out on this when using visual studio and cmake 3.8.0
             cxx_auto_type
          )