Tweak the generator pruning code that that generators with empty source

type list are considered to accept anything and never pruned.


[SVN r28103]

tools/build

@@ -1 +1 @@
-Subproject commit 415043ebdba03b5983f16706beb1505ccd3ea5df
+Subproject commit 6c8695e4028ae3793ddccfbccfa6522f4088528b