Both meson and autotools were stripping g_return_if_fail calls in the
preprocessor. This meant that when mozo hit a NULL tree root during menu
reload, the guard in matemenu_tree_item_ref did nothing and it crashed
instead of returning gracefully.

We change the guards to direct NULL checks and enable debug mode as the
default to fix this.

Fixes mate-desktop/mozo#90

PR #120 fixed the autotools build to conditionally include GioUnix-2.0
in GIR scanning when gio-unix-2.0 >= 2.80 is present, but the meson
build was not updated. Apply the equivalent fix.

Bumps [actions/checkout](https://github.com/actions/checkout) from 6 to 7.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v6...v7)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Those directories have been deprecated for over a decade and GIO's
GDesktopAppInfo API already ignores them ...
It's time to let go, simply ignore any <LegacyKDEDirs> and <LegacyDir>
elements in menus.

Backported from https://gitlab.gnome.org/GNOME/gnome-menus/-/commit/13c6790a

Fixes #108