Turn `libpam-sys-consts` back into `libpam-sys-impls`. This moves the constants into `libpam-sys` and makes `libpam-sys-impls` responsible solely for detecting the current PAM implementation.

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