SetGPUArchs updated to work around a CMake FindCUDAToolkit issue (rapidsai#695)

CMake < 3.20 has a bug in FindCUDAToolkit where it won't properly detect the `CUDAToolkit` version when `find_package(CUDAToolkit)` occurs before `enable_language(CUDA)`

Authors:
  - Robert Maynard (@robertmaynard)
  - Mike Wendt (@mike-wendt)

Approvers:
  - Keith Kraus (@kkraus14)
  - Mark Harris (@harrism)

URL: rapidsai#695

Author: robertmaynard

cmake/Modules/SetGPUArchs.cmake

@@ -25,15 +25,28 @@ else()
   list(REMOVE_ITEM SUPPORTED_CUDA_ARCHITECTURES "62" "72")
 endif(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
 
+# CMake < 3.20 has a bug in FindCUDAToolkit where it won't properly detect the CUDAToolkit version
+# when find_package(CUDAToolkit) occurs before enable_language(CUDA)
+if(NOT DEFINED CUDAToolkit_VERSION AND CMAKE_CUDA_COMPILER)
+  execute_process(COMMAND ${CMAKE_CUDA_COMPILER} "--version" OUTPUT_VARIABLE NVCC_OUT)
+  if(NVCC_OUT MATCHES [=[ V([0-9]+)\.([0-9]+)\.([0-9]+)]=])
+    set(CUDAToolkit_VERSION_MAJOR "${CMAKE_MATCH_1}")
+    set(CUDAToolkit_VERSION_MINOR "${CMAKE_MATCH_2}")
+    set(CUDAToolkit_VERSION_PATCH "${CMAKE_MATCH_3}")
+    set(CUDAToolkit_VERSION "${CMAKE_MATCH_1}.${CMAKE_MATCH_2}.${CMAKE_MATCH_3}")
+  endif()
+  unset(NVCC_OUT)
+endif()
+
 if(CUDAToolkit_VERSION_MAJOR LESS 11)
   list(REMOVE_ITEM SUPPORTED_CUDA_ARCHITECTURES "80")
-endif(CUDAToolkit_VERSION_MAJOR LESS 11)
+endif()
 if(CUDAToolkit_VERSION_MAJOR LESS 10)
   list(REMOVE_ITEM SUPPORTED_CUDA_ARCHITECTURES "75")
-endif(CUDAToolkit_VERSION_MAJOR LESS 10)
+endif()
 if(CUDAToolkit_VERSION_MAJOR LESS 9)
   list(REMOVE_ITEM SUPPORTED_CUDA_ARCHITECTURES "70")
-endif(CUDAToolkit_VERSION_MAJOR LESS 9)
+endif()
 
 # If `CMAKE_CUDA_ARCHITECTURES` is not defined, build for all supported architectures. If
 # `CMAKE_CUDA_ARCHITECTURES` is set to an empty string (""), build for only the current