Pass the git version string to tiff2png.c when compiling.

This doesn't solve the problem of how to embed a correct
version string in published tarballs; they'll just use
the unknown version fallback. We need something more
sophisticated here.
Set a fallback version in the makefile for tarballs.

Use this when git isn't available, either because
the command fails or because we're building from
a source package instead of a repository.