Add probe_sysroot_crate and emit_sysroot_crate

Review

Review (pt. 2)

src/lib.rs

@@ -212,6 +212,25 @@ impl AutoCfg {
         Ok(status.success())
     }
 
+    /// Tests whether the given sysroot crate can be used.
+    ///
+    /// The test code is subject to change, but currently looks like:
+    ///
+    /// ```ignore
+    /// extern crate CRATE as probe;
+    /// ```
+    pub fn probe_sysroot_crate(&self, name: &str) -> bool {
+        self.probe(format!("extern crate {} as probe;", name)) // `as _` wasn't stabilized until Rust 1.33
+            .unwrap_or(false)
+    }
+
+    /// Emits a config value `has_CRATE` if `probe_sysroot_crate` returns true.
+    pub fn emit_sysroot_crate(&self, name: &str) {
+        if self.probe_sysroot_crate(name) {
+            emit(&format!("has_{}", mangle(name)));
+        }
+    }
+
     /// Tests whether the given path can be used.
     ///
     /// The test code is subject to change, but currently looks like:

src/tests.rs

@@ -56,6 +56,25 @@ fn probe_sum() {
     assert!(missing ^ ac.probe_type("std::iter::Sum<i32>"));
 }
 
+#[test]
+fn probe_std() {
+    let ac = AutoCfg::with_dir("target").unwrap();
+    assert_eq!(ac.probe_sysroot_crate("std"), !ac.no_std);
+}
+
+#[test]
+fn probe_alloc() {
+    let ac = AutoCfg::with_dir("target").unwrap();
+    let missing = !ac.probe_rustc_version(1, 36);
+    assert!(missing ^ ac.probe_sysroot_crate("alloc"));
+}
+
+#[test]
+fn probe_bad_sysroot_crate() {
+    let ac = AutoCfg::with_dir("target").unwrap();
+    assert!(!ac.probe_sysroot_crate("doesnt_exist"));
+}
+
 #[test]
 fn probe_no_std() {
     let ac = AutoCfg::with_dir("target").unwrap();