meson: add corresponding target for Makefile's hdr-check

To bridge the remaining gaps between Makefile and meson, this patch series adds a new check 'headers-check' which is similar to the Makefile's 'hdr-check'.

The first two commits are small cleanups, where we re-organize existing variables to make it easier to add the target. The third commit adds the target.

This is based on master 5b97a56f (The second batch, 2025-03-29), with 'es/meson-build-skip-coccinelle' merged in.