Don't crash if the rustc version can't be determined

This was causing troubles with `cargo clippy` as they seem to replace
the RUSTC env variable (see https://github.com/Manishearth/rust-clippy/issues/1517)

build.rs

@@ -1,27 +1,41 @@
 use std::env;
 use std::ffi::OsString;
 use std::process::Command;
+use std::error::Error;
 
 
-fn get_rustc_version() -> (i32, i32) {
+fn get_rustc_version() -> Result<(i32, i32), Box<Error>> {
+    fn err() -> Box<Error> {
+        "Can't determine the rustc version.".into()
+    }
+
     let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
     let out = String::from_utf8(Command::new(&rustc)
         .arg("--version")
         .output()
         .unwrap().stdout).unwrap();
     let mut pieces = out.split(' ');
-    let _ = pieces.next();
+    let _ = pieces.next().ok_or(err())?;
 
-    let mut ver_iter = pieces.next().unwrap().split('.').map(|x| x.parse().unwrap());
-    (ver_iter.next().unwrap(), ver_iter.next().unwrap())
+    let mut ver_iter = pieces
+        .next()
+        .ok_or(err())?
+        .split('.')
+        .map(|x| x.parse());
+    let major = ver_iter.next().ok_or(err())??;
+    let minor = ver_iter.next().ok_or(err())??;
+    Ok((major, minor))
 }
 
 fn rustc_has_unix_socket() -> bool {
     if !cfg!(unix) {
         false
     } else {
-        let (major, minor) = get_rustc_version();
-        major > 1 || (major == 1 && minor >= 10)
+        if let Ok((major, minor)) = get_rustc_version() {
+            major > 1 || (major == 1 && minor >= 10)
+        } else {
+            false
+        }
     }
 }