Generate version.rs from librsvg/build.rs
This fixes the build pipeline which was broken after the merge of !386 (merged).
Now we parse the version numbers from configure.ac and create version.rs from build.rs. This works better with builds where build_dir != src_dir.
Based on code kindly suggested by user notriddle at https://users.rust-lang.org/
Edited by Sven Neumann