Hide C compiler warnings about unused items, explain some others #994

Job	Run time
OS (apt-docker.io/amd64/ubuntu:devel)	1m 49s
OS (apt-docker.io/amd64/ubuntu:latest)	2m 19s
OS (apt-docker.io/amd64/ubuntu:rolling)	1m 58s
OS (apt-docker.io/i386/debian:testing)	1m 46s
OS (apt-docker.io/amd64/debian:testing)	1m 56s
OS (dnf-registry.fedoraproject.org/fedora:latest)	2m 38s
OS (dnf-registry.fedoraproject.org/fedora:rawhide)	2m 8s
OS (alpine-docker.io/alpine)	30s
OS (alpine-docker.io/i386/alpine)	33s
OS ([email protected]/alpine)	27s
OS (nix-docker.io/nixos/nix)	59s
OS (gentoo-)	5m 22s
OS (alpine@gudev-alpine, EXTRA_PACKAGES=libgudev-dev)	32s
	22m 57s

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