code: def VariantDir(self, variant_dir, src_dir, duplicate=1):
          
  
          if not isinstance(src_dir, SCons.Node.Node):
              src_dir = self.Dir(src_dir)
          if not isinstance(variant_dir, SCons.Node.Node):
              variant_dir = self.Dir(variant_dir)
          if src_dir.is_under(variant_dir):
              raise SCons.Errors.UserError("Source directory cannot be under variant directory.")
          if variant_dir.srcdir:
              if variant_dir.srcdir == src_dir:
                  return # We already did this.
              raise SCons.Errors.UserError("'%s' already has a source directory: '%s'."%(variant_dir, variant_dir.srcdir))
          variant_dir.link(src_dir, duplicate)
docs: Link the supplied variant directory to the source directory
          for purposes of building files.