Merge topic 'FetchContent-SYSTEM'

4b85141f83 FetchContent: Don't pass SYSTEM through to sub-build

Acked-by: Kitware Robot <[email protected]>
Merge-request: !7977

Modules/FetchContent.cmake

@@ -1425,6 +1425,9 @@ function(__FetchContent_directPopulate contentName)
 
   set(options
       QUIET
+      # SYSTEM has no meaning for ExternalProject, it is only used by us in
+      # FetchContent_MakeAvailable(). We need to parse and discard it here.
+      SYSTEM
   )
   set(oneValueArgs
       SUBBUILD_DIR