Building specific products fails if some products wont be build

Hello,

the bug occurres for me when I want to build with commandline on a CI system.
My project splits its libraries in its functional parts (the library need for deployment) and libraries containing mocks for tests. For mocking I use gmock and wrote a own product on which all mock libraries depend. In release configuration the gmock product disable itself (`condition: qbs.buildVariant === "debug").
If I build the project with `qbs build -p foo,bar,foobar config:release` it will not build and outputs: "ERROR: /my/foobar/path4:1 Product 'foobar Mocks' depends on 'Google Mock', /my/gmock/path:5:1 but product 'Google Mock' is disabled."

The products to build do not depend on the product 'Google Mock'. I know that because with QtCreator the products can be build if `Google Mock` is disabled.

I added a example project. It wont build with `qbs build -p mylib config:release`.

PS: using Qbs with CI is a huge pain because the commandline frontend does not behave like QtCreator. See https://bugreports.qt.io/browse/QBS-1392.