source files can be filtered on multiple dependencies

I need something like this for the libmpv work I'm doing; basically supporting this form in `wscript_build.py`:

```python
        ( "osdep/ar/HIDRemote.m",                "cocoa" ),
        ( "osdep/macosx_application.m",          "cocoa", "!libmpv-shared" ),
        ( "osdep/macosx_events.m",               "cocoa" ),
        ( "osdep/path-macosx.m",                 "cocoa" ),
```

There are other ways this could be accomplished, like ifdefs or defining meta-dependencies or something? Anyway, this seems the most logical to me.