Check if we are in the git repo (or perhaps in a tarball)

[davidgfnet]

This makes the script skip the build.txt generation part if it's not in a git repo (for instance when downloading the repo as a tarball or similar).

[sharkwouter]

This is a good idea. What is a bit of a bother is that we do have this same code in most of our repos, so we should apply it to them as well.