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.

FROM docker.io/debian:stable

RUN apt-get -y update \
    && apt-get -y upgrade \
    && apt-get install -y cargo mercurial \
    && apt-get clean