FindCUDA: Only depend on Threads::Threads on platforms that need it

In commit 46371132b3 (FindCUDA: CUDA_LIBRARIES doesn't contain raw
`-pthread`, 2019-11-11, v3.17.0-rc1~455^2) we introduced use of the
`Threads::Threads` target, but we do not `find_package(Threads)` on all
platforms.  Use the target only if it exists.

Modules/FindCUDA.cmake

@@ -929,7 +929,11 @@ set(CUDA_LIBRARIES)
 if(CUDA_BUILD_EMULATION AND CUDA_CUDARTEMU_LIBRARY)
   list(APPEND CUDA_LIBRARIES ${CUDA_CUDARTEMU_LIBRARY})
 elseif(CUDA_USE_STATIC_CUDA_RUNTIME AND CUDA_cudart_static_LIBRARY)
-  list(APPEND CUDA_LIBRARIES ${CUDA_cudart_static_LIBRARY} Threads::Threads ${CMAKE_DL_LIBS})
+  list(APPEND CUDA_LIBRARIES ${CUDA_cudart_static_LIBRARY})
+  if (TARGET Threads::Threads)
+    list(APPEND CUDA_LIBRARIES Threads::Threads)
+  endif()
+  list(APPEND CUDA_LIBRARIES ${CMAKE_DL_LIBS})
   if (CUDA_rt_LIBRARY)
     list(APPEND CUDA_LIBRARIES ${CUDA_rt_LIBRARY})
   endif()