Allow builds from tarballs

[eddelbuettel]

The Makefile currently assumes

1. that it is running from a git repo
2. that the git binary is present

neither one of which is true in batched / headless builds off a release tarball. I can think of one or two ways to generalize this. Maybe the simplest / quirkiest would just be to make the Makefile self-referencing where version is a constant but you add an (optional) target to update it. But not using := it should not affect other make targets. (Untested)