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).