Motivation for this change

We are patching g-ir-scanner to produce absolute paths in the GIR files. When an application uses an internal library placed in a non-standard path (e.g. $out/lib/gnome-shell), the scanner needs to be informed. For autotools-based apps, the full path was obtained from libtool-wrapped files; with Meson, this is no longer possible – we need to pass the path information in some other way.

This commit channels the --fallback-library-path option added to g-ir-scanner in aforementioned patch.

This is part of GNOME 3.26 update (#29392) and it is required for GNOME Shell and Polari.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

cc @aszlig (author of --fallback-library-path option in the gobjectIntospection patch)
cc @mbe, @rasendubi (meson maintainers)