Use standard #[cfg] directives rather than custom proc macros. Instead of having to do a bunch of custom parsing and other logic that tools often choke on, this change introduces an easy way to depend upon custom #[cfg]s provided by the libpam-sys crate.

use libpam_sys::pam_impl;

fn main() {
    println!("{}", pam_impl::enable_pam_impl_cfg())
}