gnome2.gtksourceview: resolve missing AM_GNU_GETTEXT_VERSION build failure

[xworld21]

Resolve a Darwin build failure due to gettext m4 macros not being picked up.

The normal fix would be to add automake as build input, but gettext puts those files in a nonstandard folder anyway (share/gettext/m4 rather than share/aclocal), so we have to modify ACLOCAL_PATH manually.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.
• Follows the automation/AI policy.

[vcunat]

Looks like a duplicate of #521903

[phibkro]

Built successfully on x86_64-darwin (nix-build -A gnome2.gtksourceview), producing gtksourceview-2.10.5.

For cross-reference, the alternative fix in #521903 (the autoreconf approach) also builds cleanly on x86_64-darwin. Both fixes resolve the failure, so dropping the package (#522127) doesn't appear necessary on build grounds — it's a question of which approach maintainers prefer.

_{Disclosure: build verification performed with an AI agent (Claude Code, claude-opus-4-8) on x86_64-darwin; results reviewed by me.}