The following code is compiling with command: qbs build -p bar

import qbs

Project  {
    CppApplication {
        name: "foo"
        files: ["*.c"]
    }
    CppApplication {
        name: "bar"
        files: ["bar/*.c"]
        Depends {
            name: "foo"
            required: false //true
        }
    }
}

If required is false, foo isn't compiled at all, foo.present is false and exported properties isn't affect dependent product.
If required is true, foo is compiled and foo.present is undefined.

Documentation doesn't state that required and present properties should works with product dependencies, but it would be useful, e.g. for implementing configuration tests.