Replace any existing build/host symlink

This has two advantages:
1. If `build.build` changes between runs, the symlink is no longer
   silently wrong.
2. If the entire build directory is moved, the symlink is no longer
   broken because it points to the wrong absolute path.

Author: jyn514

src/bootstrap/lib.rs

@@ -21,7 +21,6 @@ use std::collections::{HashMap, HashSet};
 use std::env;
 use std::fs::{self, File};
 use std::io;
-use std::io::ErrorKind;
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 use std::str;
@@ -505,16 +504,18 @@ impl Build {
         let build_triple = build.out.join(&build.build.triple);
         t!(fs::create_dir_all(&build_triple));
         let host = build.out.join("host");
-        if let Err(e) = symlink_dir(&build.config, &build_triple, &host) {
-            if e.kind() != ErrorKind::AlreadyExists {
-                panic!(
-                    "symlink_dir({} => {}) failed with {}",
-                    host.display(),
-                    build_triple.display(),
-                    e
-                );
-            }
-        }
+        if host.is_symlink() {
+            // Left over from a previous build; overwrite it.
+            // This matters if `build.build` has changed between invocations.
+            #[cfg(windows)]
+            t!(fs::remove_dir(&host));
+            #[cfg(not(windows))]
+            t!(fs::remove_file(&host));
+        }
+        t!(
+            symlink_dir(&build.config, &build_triple, &host),
+            format!("symlink_dir({} => {}) failed", host.display(), build_triple.display())
+        );
 
         build
     }