Don't include cudnn_include_path until we find it.

dlib/dnn/test_for_cudnn/CMakeLists.txt

@@ -3,13 +3,12 @@ cmake_minimum_required(VERSION 2.8.4)
 project(cudnn_test)
 include(../../use_cpp_11.cmake)
 
-include_directories(${cudnn_include})
-
 find_package(CUDA 7.0 REQUIRED)
 set(CUDA_HOST_COMPILATION_CPP ON)
 list(APPEND CUDA_NVCC_FLAGS "-arch=sm_30;-std=c++11;-D__STRICT_ANSI__")
 add_definitions(-DDLIB_USE_CUDA)
 
 include(find_cudnn.txt)
+include_directories(${cudnn_include})
 
 cuda_add_library(cudnn_test STATIC ../cudnn_dlibapi.cpp ${cudnn} )