Improved cmake warning messages about cuDNN

dlib/CMakeLists.txt

@@ -507,11 +507,12 @@ if (NOT TARGET dlib)
          else()
             set(DLIB_USE_CUDA OFF CACHE STRING ${DLIB_USE_BLAS_STR} FORCE )
             toggle_preprocessor_switch(DLIB_USE_CUDA)
+            if (COMPILER_CAN_DO_CPP_11)
                if (NOT cudnn OR NOT cudnn_include OR NOT cudnn_test_compile_worked)
                   message(STATUS "*** cuDNN V5.0 OR GREATER NOT FOUND.  DLIB WILL NOT USE CUDA. ***")
                   message(STATUS "*** If you have cuDNN then set CMAKE_PREFIX_PATH to include cuDNN's folder.")
                endif()
-            if (NOT COMPILER_CAN_DO_CPP_11)
+            else ()
                message(STATUS "*** Dlib CUDA support requires C++11 but your compiler doesn't support it. ***")
             endif()
          endif()