Abstract the version number in the Rust codebase

[brson]

The version number is repeated many times in the Rust repo, in Makefile.in, in rust linkage and use directives, and in the docs.

[z0w0]

What would be the best way to do this? I can't think of a nice way that doesn't involve having two copies of the source tree.

[brson]

There's no language mechanism that exists yet to make this possible and it may not be worth applying a preprocessor to all our source code just to do this.

[z0w0]

If macros supported literals this could be solved using: #[link(vers = cfg!("vers"))];

[alexcrichton]

Closing as a dupe of #5642. The other one has links to another bug, and it's been nominated (otherwise they're pretty much the same).