interim solution for macOS CI

I can offer an interim solution to get macOS CI up and running again until you have figured out how and who takes care of macOS CI long term.

my proposal

Move GLib's macOS CI to my Intel runner for the time being. It's a carefully configured bare-metal machine, I manage build isolation myself as I have full control over macOS CI in the projects I'm involved with. (I never went the VM route.) This would also be my non-negotiable requirement for GLib: as long as this is on my runner, nobody touches macOS CI but me, so that I am always on top of things and can guarantee that everything works as to be expected. To clarify: I'm only talking about the bits in the CI yaml as those are relevant to keeping the runner in a controlled and healthy state.

technical details

• Parallelization is available (4 slots, shared between GLib and GTK).
• The job can be as fast as 2 minutes. (Thanks to ccache.)
• I can target as low as OS X 10.9 Mavericks.
• Issues have been created for tests that I had to disable for now (see discussion below).