Correctly infer ranlib/ar for cross-gcc toolchains

A second try at alexcrichton#163.

The GCC convention is that if the toolchain is named `$target-gnu-gcc`,
then ranlib and ar will be available as `$target-gnu-gcc-ranlib` and
`$target-gnu-gcc-ar` respectively. The code as written would infer them
to be `$target-gnu-{ranlib,ar}`, which will only work if the tools from
`binutils` (which follow that convention) are on `$PATH`.

I've also updated the logic in line with the `cc` crate to check that
the inferred `AR`/`RANLIB` is actually executable before setting it as
an override.

See also rust-lang/cc-rs#736.

src/lib.rs

@@ -346,15 +346,25 @@ impl Build {
             configure.env_remove("CROSS_COMPILE");
 
             // Infer ar/ranlib tools from cross compilers if the it looks like
-            // we're doing something like `foo-gcc` route that to `foo-ranlib`
-            // as well.
+            // we're doing something like `foo-gcc` route that to `foo-gcc-ranlib`
+            // or `foo-ranlib` (for `binutils`) as well.
             if path.ends_with("-gcc") && !target.contains("unknown-linux-musl") {
-                let path = &path[..path.len() - 4];
-                if env::var_os("RANLIB").is_none() {
-                    configure.env("RANLIB", format!("{}-ranlib", path));
-                }
-                if env::var_os("AR").is_none() {
-                    configure.env("AR", format!("{}-ar", path));
+                let suffix = &path[path.len() - 4..];
+                let path = &path[..path.len() - suffix.len()];
+                for &tool in &["RANLIB", "AR"] {
+                    if env::var_os(tool).is_some() {
+                        // Respect what came before.
+                        break;
+                    }
+
+                    for &infix in &[suffix, ""] {
+                        let candidate = format!("{}{}-{}", path, infix, tool.to_lowercase());
+                        // Only choose a tool if it's actually executable
+                        if Command::new(&candidate).output().is_ok() {
+                            configure.env(tool, candidate);
+                            break;
+                        }
+                    }
                 }
             }