rust private

rustc: Add some build scripts for librustc crates This commit adds some "boilerplate" build scripts to librustc/libsyntax crates to declare dependencies on various environment variables that are configured throughout the build. Cargo recently gained the ability to depend on environment variables in build scripts which can help trigger recompilation of a crate. This should fix weird bugs where after you make a commit or a few days later you'll get weird "not built with the same compiler" errors hopefully.