Macos run from src

I have developed more changes towards running the meld project from source on macOS, but I am not sure whether it is appropriate yet to actually merge these into the main branch yet, until more testing has been done.

Could an upstream branch GNOME/meld:macos-run-from-src be created, so that the tweaks and docs are available? Then, once they have been validated more widely, we can begin merging them into main.