Made cmake error messages a little clearer

dlib/CMakeLists.txt

@@ -625,9 +625,11 @@ if (NOT TARGET dlib)
             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.")
+               if (cuda_test_compile_worked)
+                  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()
                endif()
             else ()
                message(STATUS "*** Dlib CUDA support requires C++11 but your compiler doesn't support it. ***")