standardize cmake and env CUDNN variable names

dlib/CMakeLists.txt

@@ -468,7 +468,7 @@ if (NOT TARGET dlib)
                   message(STATUS "Checking if you have the right version of cuDNN installed.")
                   try_compile(cudnn_test_compile_worked ${PROJECT_BINARY_DIR}/cudnn_test_build 
                               ${PROJECT_SOURCE_DIR}/dnn/test_for_cudnn cudnn_test
-                              CMAKE_FLAGS -Dcudnn=${cudnn} -Dcudnn_include=${cudnn_include})
+                              CMAKE_FLAGS -DCUDNN_INCLUDE_DIR=${CUDNN_INCLUDE_DIR} -DCUDNN_LIBRARY_DIR=${CUDNN_LIBRARY_DIR})
                   if (cudnn_test_compile_worked)
                      message(STATUS "Found cuDNN: " ${cudnn})
                   else()

dlib/dnn/test_for_cudnn/find_cudnn.txt

@@ -3,12 +3,12 @@ message(STATUS "Looking for cuDNN install...")
 # Look for cudnn, we will look in the same place as other CUDA
 # libraries and also a few other places as well.
 find_path(cudnn_include cudnn.h
-    HINTS ${CUDA_INCLUDE_DIRS} $ENV{CUDNN_INCLUDE_DIR}
+    HINTS ${CUDA_INCLUDE_DIRS} ${CUDNN_INCLUDE_DIR} $ENV{CUDNN_INCLUDE_DIR} 
     PATHS /usr/local/include
     )
 get_filename_component(cudnn_hint_path ${CUDA_CUBLAS_LIBRARIES} PATH)
 find_library(cudnn cudnn
-    HINTS ${cudnn_hint_path} $ENV{CUDNN_LIBRARY_DIR}
+    HINTS ${cudnn_hint_path} ${CUDNN_LIBRARY_DIR} $ENV{CUDNN_LIBRARY_DIR}
     PATHS /usr/local/lib64 
             /usr/local/cuda/lib64
             /usr/local/cuda/lib