gdk-pixbuf: support cross-compilation by disabling introspection and gtk_doc
gdk-pixbuf: support cross-compilation by disabling introspection and gtk_doc

I don't know why the gating on whether gobject-introspection is or isn't
null was added, but I removed it.

It was added here: 7fbcc562609249a6ba81b26064e31c00ed38ebda.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs