Vendor dependencies

Let's see how I like this workflow.

vendor/crc32fast/build.rs

@@ -0,0 +1,35 @@
+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()
+}