dependencies need three importance levels

the ability to make dependencies optional has been added somewhat recently.
however, this is not sufficient: some dependencies may be optional, but still strongly recommended. in practice that means that if such a dependency is not satisfied, the build should break by default, noting that the dependency may be explicitly disabled. that would happen via the mechanism outlined in QBS-574.

Depends {
    name: "OpenSSL"
    level: Recommended // Mandatory, Recommended, Optional
}