Avoid specifying a clang target triple when not cross-compiling

This avoids certain corner cases where supplying the same llvm triple used by
rust for clang++ can cause native (host == target) C++ builds to break.

This fix supersedes rust-lang#785. See relevant discussions in rust-lang#785 and rust-lang#788.

src/lib.rs

@@ -1662,7 +1662,11 @@ impl Build {
                 if !(target.contains("android")
                     && android_clang_compiler_uses_target_arg_internally(&cmd.path))
                 {
-                    if target.contains("darwin") {
+                    if self.get_host()? == target {
+                        // We are not cross-compiling so we do not need to explicitly
+                        // provide a target triple to the compiler, something which
+                        // may require a lot of extra work to get right (see #785).
+                    } else if target.contains("darwin") {
                         if let Some(arch) =
                             map_darwin_target_from_rust_to_compiler_architecture(target)
                         {