auto merge of #10970 : erickt/rust/master, r=alexcrichton

This modifies the `configure` to set `CFG_ENABLE_CLANG=1` if `gcc` is actually `clang`. This is needed because OS X's clang enables warning on unused command line arguments. Our build system suppresses this by default, but when `CFG_ENABLE_CLANGE=0`, we do not. This causes one of the tests to fail. This patch changes rust to directly use clang, which fixes the build failure.

Closes #10811