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
2 jobs for xry111/pam-blfs in 4 minutes and 19 seconds (queued for 5 seconds)
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