Cast unsigned literals to signed integers.
Cast unsigned literals to signed integers.

This makes it explicity that we know the literals are to large and
will result in a negative number. This gets rid of a warning in
1.9.0 and younger versions, as well as an error when using the
constants as discrimintants of #[repr(i32)] enumerations in older
versions.

rust-lang/libc,mneumann/libc,rust-lang/libc,rust-lang-nursery/libc,anatol/libc,zethra/libc,anatol/libc,superriva/libc,rust-lang-nursery/libc,zethra/libc,cactorium/libc,zethra/libc,anatol/libc,mneumann/libc,cactorium/libc,superriva/libc,rust-lang-nursery/libc,mneumann/libc,philippkeller/libc,philippkeller/libc,philippkeller/libc,cactorium/libc,rust-lang/libc,superriva/libc