OpenSSL-sys: Cfg off target_os instead off feature.

extern crate libc;

#[cfg(feature = "libressl-pnacl-sys")]

#[cfg(target_os = "nacl")]

extern crate "libressl-pnacl-sys" as _for_linkage;

use libc::{c_void, c_int, c_char, c_ulong, c_long, c_uint, c_uchar, size_t};