Using conditional dependencies will result in an error where those conditions are false.

For example:

import qbs 1.0

Application {
    Depends { name: "Sparkle"; condition: qbs.targetOS === "mac" }
    name: "CoolApp"
    files: "main.cpp"
}

will result in "Product CoolApp depends on Sparkle but Sparkle is disabled" on Windows, Linux, etc.