x11-wm/mangowc: Update to 0.12.3

• Add a patch to meson.build, otherwise mango always looks for the global configuration file in the /etc/mango directory (even if you pass the argument --sysconfdir to meson), instead of ${PREFIX}/etc/mango.

ChangeLog: https://github.com/DreamMaoMao/mangowc/releases/tag/0.12.3
Reported by: DreamMaoMao <notifications@github.com>