Changed the cmake scripts to forward CMAKE_PREFIX_PATH to the

try_compile statements rather than a cuDNN specific variable.

dlib/CMakeLists.txt

@@ -455,23 +455,27 @@ if (NOT TARGET dlib)
             # linux).
             list(APPEND CUDA_NVCC_FLAGS "-arch=sm_30;-std=c++11;-D__STRICT_ANSI__")
 
-            set(CUDNN_INCLUDE_DIR "" CACHE PATH "Directory in which to look for cudnn include files")
-            set(CUDNN_LIBRARY_DIR "" CACHE PATH "Directory in which to look for cudnn library")
-
             include(dnn/test_for_cudnn/find_cudnn.txt)
 
             if (cudnn AND NOT DEFINED cuda_test_compile_worked AND NOT DEFINED cudnn_test_compile_worked)
                # make sure cuda is really working by doing a test compile
                message(STATUS "Building a CUDA test project to see if your compiler is compatible with CUDA...")
                try_compile(cuda_test_compile_worked ${PROJECT_BINARY_DIR}/cuda_test_build 
-                           ${PROJECT_SOURCE_DIR}/dnn/test_for_cuda cuda_test)
+                           ${PROJECT_SOURCE_DIR}/dnn/test_for_cuda cuda_test
+                           CMAKE_FLAGS "-DCMAKE_PREFIX_PATH=${CMAKE_PREFIX_PATH}"
+                                       "-DCMAKE_INCLUDE_PATH=${CMAKE_INCLUDE_PATH}"
+                                       "-DCMAKE_LIBRARY_PATH=${CMAKE_LIBRARY_PATH}"
+                           )
                if (NOT cuda_test_compile_worked)
                   message(STATUS "*** CUDA was found but your compiler failed to compile a simple CUDA program so dlib isn't going to use CUDA. ***")
                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 
                               ${PROJECT_SOURCE_DIR}/dnn/test_for_cudnn cudnn_test
-                              CMAKE_FLAGS -DCUDNN_INCLUDE_DIR=${CUDNN_INCLUDE_DIR} -DCUDNN_LIBRARY_DIR=${CUDNN_LIBRARY_DIR})
+                              CMAKE_FLAGS "-DCMAKE_PREFIX_PATH=${CMAKE_PREFIX_PATH}"
+                                          "-DCMAKE_INCLUDE_PATH=${CMAKE_INCLUDE_PATH}"
+                                          "-DCMAKE_LIBRARY_PATH=${CMAKE_LIBRARY_PATH}"
+                                          )
                   if (cudnn_test_compile_worked)
                      message(STATUS "Found cuDNN: " ${cudnn})
                   else()
@@ -501,6 +505,7 @@ if (NOT TARGET dlib)
             toggle_preprocessor_switch(DLIB_USE_CUDA)
             if (NOT cudnn OR NOT cudnn_include OR NOT cudnn_test_compile_worked)
                message(STATUS "*** cuDNN V4.0 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()
             if (NOT COMPILER_CAN_DO_CPP_11)
                message(STATUS "*** Dlib CUDA support requires C++11 but your compiler doesn't support it. ***")

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