support --with-os-type=lfs and /etc/lfs-release autodetection

This MR makes polkit build system to try to recognize LFS system and customize PAM configuration for it. It's a LFS specific change so it won't affect others, but make LFS builders' work slightly eaiser.

See: http://www.linuxfromscratch.org/blfs/view/systemd/postlfs/polkit.html