Use autocfg to probe for i128

Cargo.toml

@@ -60,3 +60,6 @@ version = "1.0"
 default = ["std"]
 i128 = ["num-integer/i128", "num-traits/i128"]
 std = ["num-integer/std", "num-traits/std"]
+
+[build-dependencies]
+autocfg = "0.1.2"

build.rs

@@ -1,35 +1,14 @@
+extern crate autocfg;
+
 use std::env;
-use std::io::Write;
-use std::process::{Command, Stdio};
 
 fn main() {
-    if probe("fn main() { 0i128; }") {
+    let ac = autocfg::new();
+    if ac.probe_type("i128") {
         println!("cargo:rustc-cfg=has_i128");
     } else if env::var_os("CARGO_FEATURE_I128").is_some() {
         panic!("i128 support was not detected!");
     }
-}
-
-/// Test if a code snippet can be compiled
-fn probe(code: &str) -> bool {
-    let rustc = env::var_os("RUSTC").unwrap_or_else(|| "rustc".into());
-    let out_dir = env::var_os("OUT_DIR").expect("environment variable OUT_DIR");
-
-    let mut child = Command::new(rustc)
-        .arg("--out-dir")
-        .arg(out_dir)
-        .arg("--emit=obj")
-        .arg("-")
-        .stdin(Stdio::piped())
-        .spawn()
-        .expect("rustc probe");
-
-    child
-        .stdin
-        .as_mut()
-        .expect("rustc stdin")
-        .write_all(code.as_bytes())
-        .expect("write rustc stdin");
 
-    child.wait().expect("rustc probe").success()
+    autocfg::rerun_path(file!());
 }