Switch default PAM detection strategy to target-based. To make cross-compilation easier (like for docs.rs), this change makes OS-based detection of PAM the default, only falling back to probing the actual installed PAM as a last resort. I haven't been able to find a Linux distribution that uses anything but Linux-PAM.

# `libpam-sys-helpers`: Cross-platform helpers for `libpam-sys`

This provides memory-management helpers for `libpam-sys`.

This does not actually depend upon `libpam-sys`; it only uses `libpam-sys-consts`.