Make possible to build against gtk3, which is useful for HiDPI displays.

Motivation for this change

audacity looks pretty bad with HiDPI scaling on gtk2. This change removes hard dependency on gtk2, which should prevent build failure when wxWidgets is overridden to build against gtk3.

* The option to use gtk3 is not very obvious. Maybe we should use an alternative approach, such as a withGtk3 option that overrides the gtk version in wxWidgets?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @the-kenny