Provide a way to inject build settings for the whole project

[lacasseio]

Some Xcode build requires external custom paths to be provided to Xcode, via Preferences -> Locations -> Custom Paths. Without those values, some paths may not resolve correctly. We should introduce a way to add additional build settings. Adding build settings per task may be enough for this feature given we could do the following in settings.gradle:

gradle.allprojects {
    tasks.withType(XcodebuildExec).configureEach {
        buildSetting 'MY_PATH', file('some/path')
    }
}