In case the temporary director is not deleted, this makes it easy to
find. It also means that it will get removed when `cargo clean` removes
the target directory.
Right now we add them before, and we add -Wall by default. This allows
users to manually turn off specific warnings, without having to resort
to a dance of:
```
.warnings(false)
.flag("-Wall")
.flag("-Wno-foo")
```
This strikes a good balance of keeping -Wall on by default, while
minimising the surprising experience observed in
https://github.com/alexcrichton/cc-rs/issues/235.