Include Cargo rustc configuration in probe

Without this, environments that configure `RUSTFLAGS` or the
`RUSTC_WRAPPER` in ways that break compilation with `backtrace` will
fail to compile anyhow (see dtolnay#156).

With this, the compiler probe takes into account that Cargo
configuration, and thus (more) accurately represents whether the
`backtrace` feature can be safely enabled.

Requires rust-lang/cargo#9601, but does not break without.

Fixes dtolnay#156.

build.rs

@@ -67,17 +67,37 @@ fn compile_probe() -> Option<ExitStatus> {
     let out_dir = env::var_os("OUT_DIR")?;
     let probefile = Path::new(&out_dir).join("probe.rs");
     fs::write(&probefile, PROBE).ok()?;
-    Command::new(rustc)
-        .stderr(Stdio::null())
+
+    // Make sure to pick up Cargo rustc configuration.
+    let mut cmd = env::var_os("CARGO_RUSTC_WRAPPER").map_or_else(
+        || Command::new(&rustc),
+        |wrapper| {
+            let mut cmd = Command::new(wrapper);
+            // The wrapper's first argument should always be the path to rustc.
+            cmd.arg(&rustc);
+            cmd
+        },
+    );
+
+    cmd.stderr(Stdio::null())
         .arg("--edition=2018")
         .arg("--crate-name=anyhow_build")
         .arg("--crate-type=lib")
         .arg("--emit=metadata")
         .arg("--out-dir")
         .arg(out_dir)
-        .arg(probefile)
-        .status()
-        .ok()
+        .arg(probefile);
+
+    // If Cargo wants to set RUSTFLAGS, use that.
+    if let Ok(rustflags) = env::var("CARGO_ENCODED_RUSTFLAGS") {
+        if !rustflags.is_empty() {
+            for arg in rustflags.split('\x1f') {
+                cmd.arg(arg);
+            }
+        }
+    }
+
+    cmd.status().ok()
 }
 
 fn rustc_minor_version() -> Option<u32> {