meson: support -Dos_type=lfs and /etc/lfs-release autodetection

This makes polkit building 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 it makes LFS builders' work slightly eaiser.

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

Superseds !57 (closed).