Config tweaks round 2 (#12518)

Here's the new dependency graph in `--qtdeps=system` mode. (Gray edges go to parts of the C/C++ runtime; blue edges come directly from the phantomjs binary. libc6 has been suppressed entirely, as literally everything else depends on it, so it doubles the visual clutter.)  No more GStreamer.

![pjs-deps](https://cloud.githubusercontent.com/assets/325899/4083255/7682213c-2ef9-11e4-8e2f-295a4fe79f0d.png)