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

If we don't filter out SYSTEM from the arguments we pass
through to ExternalProject_Add(), it gets appended as a
list item to whatever arguments precede it because
ExternalProject_Add() doesn't recognize it as a keyword.

Fixes: #24201

Modules/FetchContent.cmake

@@ -1427,6 +1427,9 @@ 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