Authentication links don't work when built without webkitgtk

[jbicha]

Expected behavior

When gamehub is built without the webkit2gtk dependency, the authentication links use the user's default web browser instead.

Or the webkit2gtk dependency become a hard dependency.

Actual behavior

The authentication links silently don't work when gamehub is built without the webkit2gtk dependency.

Steps to reproduce

1. From Ubuntu or Debian, build without libwebkit2gtk-4.0-dev installed.
2. Open Gamehub
3. On the "All your games in one place Let's get started" screen, click the GOG Authentication required or Humble Bundle Authentication required buttons.
4. A blank dialog opens.

Version and environment

• Ubuntu 24.04 LTS (pre-release)
• gamehub 0.16.3-2-master+ds-1