xcconfig cleanup

There was a bit of duplication in them, so I moved more stuff to Common.xcconfig.
Also lots of redundant commented out stuff, like...

```
// Disabled for now, plugins get angry
//GCC_ENABLE_OBJC_GC = supported
```

Good job we didn't uncomment it eh @tiennou ? :)

I also added stuff that Xcode 4.3 is complaining about in plugins to 'validate' the project.