Draft: d/rules: Opt-in to cross-friendly gobject-introspection, vapigen wrappers

Requires vala!3 in Vala for working cross-builds (native builds don't need that change).

• d/rules: Opt-in to cross-friendly gobject-introspection wrappers

  This workaround for #1060838 is another step towards making cross-compiling possible. (It is necessary, but not sufficient: #1061107 needs to be resolved in vala before we will actually be able to cross-compile gcr4.)

• d/rules: Use host architecture vapigen

  This will make it possible to cross-compile gcr4 after #1061107 is fixed in vala.

• Update changelog