Make cuDNN test project failure print a message saying exactly why it failed.

dlib/CMakeLists.txt

@@ -687,19 +687,27 @@ if (NOT TARGET dlib)
                   message(STATUS "*****************************************************************************************************************")
                   message(STATUS "*** CUDA was found but your compiler failed to compile a simple CUDA program so dlib isn't going to use CUDA. ")
                   message(STATUS "*** The output of the failed CUDA test compile is shown below: ")
+                  message(STATUS "*** ")
                   message(STATUS "***   ${try_compile_output_message}")
                   message(STATUS "*****************************************************************************************************************")
                else()
-                  message(STATUS "Checking if you have the right version of cuDNN installed.")
+                  message(STATUS "Building a cuDNN test project to check if you have the right version of cuDNN installed...")
                   try_compile(cudnn_test_compile_worked 
                      ${PROJECT_BINARY_DIR}/cudnn_test_build 
                      ${PROJECT_SOURCE_DIR}/cmake_utils/test_for_cudnn cudnn_test
                      CMAKE_FLAGS ${CUDA_TEST_CMAKE_FLAGS}
+                     OUTPUT_VARIABLE try_compile_output_message
                      )
                   if (NOT cudnn_test_compile_worked)
-                     message(STATUS "*** Found cuDNN, but it looks like the wrong version so dlib will not use it.              ***")
-                     message(STATUS "*** Dlib requires cuDNN V5.0 OR GREATER.  Since cuDNN is not found DLIB WILL NOT USE CUDA. ***")
-                     message(STATUS "*** If you have cuDNN then set CMAKE_PREFIX_PATH to include cuDNN's folder.                ***")
+                     string(REPLACE "\n" "\n   ***   " try_compile_output_message "${try_compile_output_message}")
+                     message(STATUS "*****************************************************************************************************")
+                     message(STATUS "*** Found cuDNN, but we failed to compile the dlib/cmake_utils/test_for_cudnn project. ")
+                     message(STATUS "*** You either have an unsupported version of cuDNN or something is wrong with your cudDNN install.")
+                     message(STATUS "*** Since a functional cuDNN is not found DLIB WILL NOT USE CUDA. ")
+                     message(STATUS "*** The output of the failed test_for_cudnn build is: ")
+                     message(STATUS "*** ")
+                     message(STATUS "***   ${try_compile_output_message}")
+                     message(STATUS "*****************************************************************************************************")
                   endif()
                endif()
             endif()