Provide a way to declare dependencies as system-provided

QBS should provide a way to disable warnings from modules linked in a Depends item. For example, one is unlikely to want to see warnings in the Qt headers.

A suggested syntax might be something like:

Depends { name: "Qt.core"; system: true }

When `system` is true, the dependency would add its headers using systemIncludePaths / systemFrameworkPaths instead of includePaths / frameworkPaths.