Update submodule: libglnx 2024-04-20

[smcv]

• Fix function detection when using -Werror=return-type (Resolves: [Bug]: 1.15.7 pre-release subproject libglnx build failure in opensuse rpmbuild environment using meson due to flaw in feature detection code #5778)
• Add a fallback definition for G_PID_FORMAT
• Avoid warnings for g_steal_fd() when targeting older GLib

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Changes: https://gitlab.gnome.org/GNOME/libglnx/-/merge_requests/55, https://gitlab.gnome.org/GNOME/libglnx/-/merge_requests/56, https://gitlab.gnome.org/GNOME/libglnx/-/merge_requests/57

[smcv]

Actually, don't land this right now - I'm preparing an alternative PR which converts libglnx into a git subtree to avoid the problems with submodules that @GeorgesStavracas encountered while releasing 1.14.7, and having this MR side-by-side with its git subtree equivalent will make a useful comparison.

[smcv]

Actually, don't land this right now - I'm preparing an alternative PR which converts libglnx into a git subtree to avoid the problems with submodules that @GeorgesStavracas encountered while releasing 1.14.7, and having this MR side-by-side with its git subtree equivalent will make a useful comparison.

After doing #5800 (a one-time big merge to replace the submodule with a subtree), the subtree equivalent of this PR would be similar to smcv#1. I personally think that's a lot nicer.