auto merge of #17192 : skade/rust/fix-osx-linking, r=alexcrichton

auto merge of #17192 : skade/rust/fix-osx-linking, r=alexcrichton

Don't pass -fno-use-linker-plugin on OS X as clang does not accept it.

clang fails linking with:

```
error: linking with `cc` failed: exit code: 1
... arg list omitted...
note: clang: error: unknown argument: '-fno-use-linker-plugin' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
```

clang version:

```
$ clang -v
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.2.0
Thread model: posix
```