Tweak native-build.md example

`$(OUT_DIR)` may contain spaces, so it needs to be quoted. It also needs to be expanded by the shell, not by `make`, or any quotes/backslashes in the value will cause problems.
auto merge of #706 : kballard/cargo/patch-1, r=alexcrichton

`$(OUT_DIR)` may contain spaces, so it needs to be quoted. It also needs to be expanded by the shell, not by `make`, or any quotes/backslashes in the value will cause problems.