The build level shouldn't be deal with interpreter objects, by the time
they leave the intpreter they should be in the Meson middle layer
representaiton
We need to konw on rconfigure which options have already bee set not
just for the super project, but also for the subproject. However, using
first_invocation is not sufficient, as a reconfigure could add a new
subpproject that wasn't present before, and we need to initialize that
project's builtins.