Check CUDNN environment variables for path

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}
+    HINTS ${CUDA_INCLUDE_DIRS} $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}
+    HINTS ${cudnn_hint_path} $ENV{CUDNN_LIBRARY_DIR}
     PATHS /usr/local/lib64 
             /usr/local/cuda/lib64
             /usr/local/cuda/lib