rustbuild: Optimize build times slightly

As the entry point for building the Rust compiler, a good user experience hinges
on this compiling quickly to get to the meat of the problem. To that end use
`#[cfg]`-specific dependencies to avoid building Windows crates on Unix and drop
the `regex` crate for now which was easily replacable with some string
searching.

src/bootstrap/lib.rs

@@ -26,7 +26,6 @@ extern crate md5;
 extern crate num_cpus;
 extern crate rustc_serialize;
 extern crate toml;
-extern crate regex;
 
 use std::collections::HashMap;
 use std::env;