Resolve question_mark clippy lint in build script

    warning: this `match` expression can be replaced with `?`
       --> serde/build.rs:111:17
        |
    111 |       let rustc = match env::var_os("RUSTC") {
        |  _________________^
    112 | |         Some(rustc) => rustc,
    113 | |         None => return None,
    114 | |     };
        | |_____^ help: try instead: `env::var_os("RUSTC")?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark
        = note: `-W clippy::question-mark` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::question_mark)]`

    warning: this `match` expression can be replaced with `?`
       --> serde/build.rs:131:16
        |
    131 |       let next = match pieces.next() {
        |  ________________^
    132 | |         Some(next) => next,
    133 | |         None => return None,
    134 | |     };
        | |_____^ help: try instead: `pieces.next()?`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#question_mark

serde/build.rs

@@ -1,6 +1,6 @@
 use std::env;
 use std::process::Command;
-use std::str::{self, FromStr};
+use std::str;
 
 // The rustc-cfg strings below are *not* public API. Please let us know by
 // opening a GitHub issue if your build environment requires some way to enable
@@ -108,30 +108,12 @@ fn main() {
 }
 
 fn rustc_minor_version() -> Option<u32> {
-    let rustc = match env::var_os("RUSTC") {
-        Some(rustc) => rustc,
-        None => return None,
-    };
-
-    let output = match Command::new(rustc).arg("--version").output() {
-        Ok(output) => output,
-        Err(_) => return None,
-    };
-
-    let version = match str::from_utf8(&output.stdout) {
-        Ok(version) => version,
-        Err(_) => return None,
-    };
-
+    let rustc = env::var_os("RUSTC")?;
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = str::from_utf8(&output.stdout).ok()?;
     let mut pieces = version.split('.');
     if pieces.next() != Some("rustc 1") {
         return None;
     }
-
-    let next = match pieces.next() {
-        Some(next) => next,
-        None => return None,
-    };
-
-    u32::from_str(next).ok()
+    pieces.next()?.parse().ok()
 }