FindCUDA fix : do not unset CACHE variables on first call

This will allow to override CUDA CMake variables from first CMake call,
like cmake -DCUDA_TOOLKIT_ROOT_DIR=... -DCUDA_CUDA_LIBRARY=...

cmake/FindCUDA.cmake

@@ -529,7 +529,7 @@ endmacro()
 
 # Check to see if the CUDA_TOOLKIT_ROOT_DIR and CUDA_SDK_ROOT_DIR have changed,
 # if they have then clear the cache variables, so that will be detected again.
-if(NOT "${CUDA_TOOLKIT_ROOT_DIR}" STREQUAL "${CUDA_TOOLKIT_ROOT_DIR_INTERNAL}")
+if(DEFINED CUDA_TOOLKIT_ROOT_DIR_INTERNAL AND (NOT "${CUDA_TOOLKIT_ROOT_DIR}" STREQUAL "${CUDA_TOOLKIT_ROOT_DIR_INTERNAL}"))
   unset(CUDA_TARGET_TRIPLET CACHE)
   unset(CUDA_TOOLKIT_TARGET_DIR CACHE)
   unset(CUDA_NVCC_EXECUTABLE CACHE)
@@ -537,8 +537,8 @@ if(NOT "${CUDA_TOOLKIT_ROOT_DIR}" STREQUAL "${CUDA_TOOLKIT_ROOT_DIR_INTERNAL}")
   cuda_unset_include_and_libraries()
 endif()
 
-if(NOT "${CUDA_TARGET_TRIPLET}" STREQUAL "${CUDA_TARGET_TRIPLET_INTERNAL}" OR
-   (DEFINED CUDA_TOOLKIT_TARGET_DIR AND NOT "${CUDA_TOOLKIT_TARGET_DIR}" STREQUAL "${CUDA_TOOLKIT_TARGET_DIR_INTERNAL}"))
+if(DEFINED CUDA_TARGET_TRIPLET_INTERNAL AND (NOT "${CUDA_TARGET_TRIPLET}" STREQUAL "${CUDA_TARGET_TRIPLET_INTERNAL}") OR
+   (DEFINED CUDA_TOOLKIT_TARGET_DIR  AND DEFINED CUDA_TOOLKIT_TARGET_DIR_INTERNAL AND NOT "${CUDA_TOOLKIT_TARGET_DIR}" STREQUAL "${CUDA_TOOLKIT_TARGET_DIR_INTERNAL}"))
   cuda_unset_include_and_libraries()
 endif()