Reenable ability of generators to return property-set as first item in result list

- Fixes issue introduced in ee613a6a28 (feature relevance)

src/build/generators.jam

@@ -475,7 +475,13 @@ class generator
         }
         if $(result)
         {
-            return $(relevant) $(result) ;
+            if [ class.is-a $(result[1]) : property-set ]
+            {
+                return [ $(result[1]).add $(relevant) ] $(result[2-]) ;
+            }
+            else {
+               return $(relevant) $(result) ;
+            }
         }
     }