gi: support gi generation with subprojects
GI bindings are not created when gobject-introspection is a subproject dependency, for example when building in Windows.
Before finding the 'gi-ir-scanner' program, try to resolve the 'gobject-introspection' dep so that it's provided by the uninstalled submodule
To work on Windows it requires
Edited by Andoni Morales Alastruey