try_compile needs cudnn cmake vars explicitly passed

dlib/CMakeLists.txt

@@ -467,7 +467,8 @@ if (NOT TARGET dlib)
                else()
                   message(STATUS "Checking if you have the right version of cuDNN installed.")
                   try_compile(cudnn_test_compile_worked ${PROJECT_BINARY_DIR}/cudnn_test_build 
-                              ${PROJECT_SOURCE_DIR}/dnn/test_for_cudnn cudnn_test)
+                              ${PROJECT_SOURCE_DIR}/dnn/test_for_cudnn cudnn_test
+                              CMAKE_FLAGS -Dcudnn=${cudnn} -Dcudnn_include=${cudnn_include})
                   if (cudnn_test_compile_worked)
                      message(STATUS "Found cuDNN: " ${cudnn})
                   else()