CI: Build freeciv-mp-gtk4 for OSX
It should be possible to build freeciv-mp-gtk4 on OSX. That would be the only environment where we can test freeciv-mp-gtk4 build at the moment.
This follows #42251
It should be possible to build freeciv-mp-gtk4 on OSX. That would be the only environment where we can test freeciv-mp-gtk4 build at the moment.
This follows #42251