[Qbs] Using ConanfileProbe with yocto cross compiling toolchain

[Richard Weickelt]

> I just stumbled across a scenario in which we use with multiple profiles for
> different cross compiling toolchains. Yocto creates a script which exports
> environment variables. We normally just extract the needed values from the
> script and set them as additional qbs profile settings.
> This leads to the problem that the ConanfileProbe just calls "conan
> install... -p..." without the needed environment variables. 
> Thus when
> install libraries they are getting compiled with the wrong toolchain/settings.
> What is the proper way of doing this? How do I set the
> environment variables? Should I set the variables in the Kit?

That would be one option, but sounds a bit inconvenient. If the environment
variables are only relevant when running the ConanfileProbe, then you would
need to:

1. Add an additionalEnvironmentVariables property to ConanfileProbe
2. Append these to the process environment in ConanfileProbe. See
   https://doc.qt.io/qbs/jsextension-process.html#setenv
3. Add an additional Probe item in your project file that extracts
   the environment variables from the script.
   Probes are currently executed in the order of their appearance in the qbs
   file, so make sure it comes first.

Would that be a solution for you? Then I would warmthly welcome a patch.

Note that instead of modifying qbs profile settings globally with your yocto
variables, you should be able to also create an intermediate Profile item in
your project files this way:

    Project {
        YoctoScriptProbe {
            id: yocto
        }
        // This profile amends the profile set by Qt Creator or on the
        // command line
        Profile {
            name: "yocto-profile"
            baseProfile: project.profile
            someModule.someProperty: yocto.someExtractedValue
        }

        // Every product now needs to specify the amended profile
        // explicitly
        Product {
            profile: "yocto-profile"
        }
    }

Hope that helps.

Richard