cmake: improve CMAKE_CUDA_COMPILER guessing on unix (#17773)

Fixes a bug in #17293 causing an infinite loop on some systems.

CMakeLists.txt

@@ -90,9 +90,10 @@ if(USE_CUDA)
   cmake_minimum_required(VERSION 3.13.2)  # CUDA 10 (Turing) detection available starting 3.13.2
   include(CheckLanguage)
   check_language(CUDA)
-  if (NOT CMAKE_CUDA_COMPILER_LOADED AND UNIX AND EXISTS "/usr/local/cuda/bin/nvcc")
-    set(CMAKE_CUDA_COMPILER "/usr/local/cuda/bin/nvcc")
-    message(WARNING "CMAKE_CUDA_COMPILER guessed: ${CMAKE_CUDA_COMPILER}")
+  if (NOT CMAKE_CUDA_COMPILER AND UNIX AND EXISTS "/usr/local/cuda/bin/nvcc")
+    set(ENV{CUDACXX} "/usr/local/cuda/bin/nvcc")
+    message(WARNING "CMAKE_CUDA_COMPILER guessed: " $ENV{CUDACXX} "\n"
+      "Please fix your cuda installation: https://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html#mandatory-post")
   endif()
   enable_language(CUDA)
   set(CMAKE_CUDA_STANDARD 11)