CMake fails build from tarball; IGRAPH_VERSION is missing.

I tried version 0.9.x tarballs from the release tags, and only 0.9.0 will build cleanly.

The rest fail on an error “…IGRAPH_VERSION is missing”.

You need to download the release tarball, not the tag snapshot automatically produced by GitHub.

https://github.com/igraph/igraph/releases/download/0.9.6/igraph-0.9.6.tar.gz