DragonFlyBSD builder cannot build rustc

[nagisa]

See http://buildbot.rust-lang.org/builders/auto-dragonflybsd-64-opt/builds/554/steps/compile/logs/stdio

rustc: x86_64-unknown-dragonfly/stage0/lib/rustlib/x86_64-unknown-dragonfly/lib/libstd
../src/libstd/sys/unix/stack_overflow.rs:48:16: 48:24 error: unresolved import `libc::SIGSTKSZ`. There is no `SIGSTKSZ` in `libc` [E0432]
../src/libstd/sys/unix/stack_overflow.rs:48                SIGSTKSZ, sighandler_t};
                                                           ^~~~~~~~

[nagisa]

cc @alexcrichton since you seem to be the maintainer of the new liblibc.

[alexcrichton]

I'm personally not a huge fan of keeping around tracking bugs for things like this, there are likely hundreds of platforms that Rust doesn't compile on! That being said this seems like something either needs to be added to libc or the submodule just needs to be updated.