meson: allow non-dist non-git builds but mark them unsupported.
Reported by Jan Tojnar as a comment to commit e4c7fc23 that builds from simple archives of the repo contents (without the .git tree) are currently broken. Well this is normal, as we only support builds from release versions or from the repository where we can extract a git hash. Any other kind of nightly build can be from any commit out of thousands and is maintenance hell. To be nice though, let's unbreak such builds, but they will be clearly marked as unsupported (warning at configure time + the extract commit hash will now be labelled "unknown (unsupported)", which will be a visible string in About and on unstable version canvases). Basically do so at your own risk. Also generate INSTALL all the time (not sure why it was only generated in non-git case).