Configure GitHub CI to build for macOS on Ravenbrook's own Mac Mini named Merlin#229

Draft

rptb1 wants to merge 2 commits intomasterfrom

branch/2023-06-02/merlin-macos-ci

Commits on Jun 2, 2023

Updating configure script to recognize macOS 13.4.
Richard Brooksby
committed
Adding macOS build to GitHub actions, running on self-hosted runner on Merlin Mac Mini.
Richard Brooksby
committed