gildas: build with gtk3, fix homepage

[ShadowRZ]

Part of #410814

https://www.iram.fr/IRAMFR/GILDAS/install.html#3---needed-tools-to-build-executables-from-sources

This package's GTK2 support is obsolescent.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • 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.

[SuperSandroBot]

❌ nixpkgs-review result

Generated using nixpkgs-review-gha (def992f)

Command: nixpkgs-review pr 545943
Commit: 4de797a367563f99b2c493871a7c0e448854afb0 (subsequent changes)
Merge: bafbcbcfdc4468bc29049811f07666518394f9bd

Logs: https://github.com/SuperSandro2000/nixpkgs-review-gha/actions/runs/30266861938/attempts/1

---

x86_64-linux

✅ 1 package built:

• gildas

---

aarch64-linux

✅ 1 package built:

• gildas

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aarch64-darwin (sandbox = relaxed)

❌ 1 package failed to build:

• gildas