crc32fast 1.2.0

Fast, SIMD-accelerated CRC32 (IEEE) checksum computation
use std::env;
use std::process::Command;
use std::str;

fn main() {
    println!("cargo:rerun-if-changed=build.rs");

    let minor = match rustc_minor_version() {
        Some(n) => n,
        None => return,
    };

    if minor >= 27 {
        println!("cargo:rustc-cfg=crc32fast_stdarchx86");
    }
}

fn rustc_minor_version() -> Option<u32> {
    macro_rules! otry {
        ($e:expr) => {
            match $e {
                Some(e) => e,
                None => return None,
            }
        };
    }
    let rustc = otry!(env::var_os("RUSTC"));
    let output = otry!(Command::new(rustc).arg("--version").output().ok());
    let version = otry!(str::from_utf8(&output.stdout).ok());
    let mut pieces = version.split('.');
    if pieces.next() != Some("rustc 1") {
        return None;
    }
    otry!(pieces.next()).parse().ok()
}