Separate libpam-sys-helpers from libpam-sys. This separates the parts of libpam-sys that don't need linking against libpam from the parts that do need to link against libpam.

#![allow(unexpected_cfgs)]

use libpam_sys_helpers::pam_impl;

fn main() {
    println!("cargo:rustc-link-lib=pam");
    pam_impl::enable_pam_impl_cfg();
}