'distclean' requires 3 traversals of project tree

The sequence:

```
   find $(POCO_BUILD) -name obj -type d -print0 | xargs -0 rm -rf
   find $(POCO_BUILD) -name .dep -type d -print0 | xargs -0 rm -rf
   find $(POCO_BUILD) -name bin -type d -print0 | xargs -0 rm -rf
```

can be refactored as:

```
   find $(POCO_BUILD) $ -name obj -o -name .dep -o -name bin $ -type d -print0 | xargs -0 rm -rf
```

and do the same in a single traversal.
