Merge pull request #3678 from cenit/dev/cenit/cudafix

enable darknet as a library also when only MSVC build tools are installed

DarknetConfig.cmake.in

@@ -10,7 +10,11 @@ if(@OpenCV_FOUND@)
 endif()
 
 if(@ENABLE_CUDA@)
-  enable_language(CUDA)
+  include(CheckLanguage)
+  check_language(CUDA)
+  if(NOT CMAKE_CUDA_COMPILER)
+    message(STATUS " --> WARNING: Unable to find native CUDA integration!")
+  endif()
   find_dependency(CUDA)
   cuda_select_nvcc_arch_flags(CUDA_ARCH_FLAGS ${CUDA_ARCHITECTURES})
   if(@CUDNN_FOUND@)