add a build script that complains about wrong nightly versions

[oli-obk]

Similar to what rocket has. Since we don't know what future rustc versions are OK, we just emit an error message if the rustc version is too old

[phansch]

I suppose that this is the build script, right? https://github.com/SergioBenitez/Rocket/blob/master/lib/build.rs

Oh and this works using the cargo build script feature, nice!

[oli-obk]

jup. we should probably read the version numbers from an extra file with include_str (instead of hardcoding in the build.rs) so we can just generate the current version number via the update_lints python script

[phansch]

Going to take a look at this now and hope to have something merge-ready by next week 👍