rustc_trans: Append to PATH instead of prepending

Currently we run sub-commands with a different PATH so they can find the bundled
`gcc.exe` that we ship with the `*-windows-gnu` host compilers. The current
logic, however, *prepends* to `PATH` which means that if the system has a `gcc`
installed it will not be used over the bundled `gcc`.

This can cause problems, however, if the system gcc is used to compile native
code and the Rust compiler then links everything with the bundled gcc. The
standard library in both situations can be subtly different (the C standard
library), and this can lead to errors such as rust-lang/flate2-rs#27.

This commit switches the ordering by appending our own tools to `PATH` instead
of prepending, so the system tools will be favored over the bundled ones (which
are intended to only be used as a last resort anyway).

Author: alexcrichton

src/librustc_trans/back/link.rs

@@ -390,15 +390,16 @@ pub fn get_ar_prog(sess: &Session) -> String {
 
 fn command_path(sess: &Session) -> OsString {
     // The compiler's sysroot often has some bundled tools, so add it to the
-    // PATH for the child.
-    let mut new_path = sess.host_filesearch(PathKind::All)
-                           .get_tools_search_paths();
-    if let Some(path) = env::var_os("PATH") {
-        new_path.extend(env::split_paths(&path));
-    }
+    // PATH for the child. Be sure that we *append* to the PATH so we favor the
+    // system tools if they're configured and otherwise just fall back to the
+    // bundled versions.
+    let path = env::var_os("PATH").unwrap_or(OsString::new());
+    let mut new_path = env::split_paths(&path).collect::<Vec<_>>();
     if sess.target.target.options.is_like_msvc {
         new_path.extend(msvc::host_dll_path());
     }
+    new_path.extend(sess.host_filesearch(PathKind::All)
+                        .get_tools_search_paths());
     env::join_paths(new_path).unwrap()
 }