Hide C compiler warnings about unused items, explain some others #995

Job	Run time
OS (apt-docker.io/amd64/ubuntu:devel)	1m 50s
OS (apt-docker.io/amd64/ubuntu:latest)	2m 13s
OS (apt-docker.io/amd64/ubuntu:rolling)	1m 48s
OS (apt-docker.io/i386/debian:testing)	1m 42s
OS (apt-docker.io/amd64/debian:testing)	2m 1s
OS (dnf-registry.fedoraproject.org/fedora:latest)	2m 44s
OS (dnf-registry.fedoraproject.org/fedora:rawhide)	2m 34s
OS (alpine-docker.io/alpine)	29s
OS (alpine-docker.io/i386/alpine)	38s
OS ([email protected]/alpine)	33s
OS (nix-docker.io/nixos/nix)	53s
OS (gentoo-)	6m 15s
OS (alpine@gudev-alpine, EXTRA_PACKAGES=libgudev-dev)	38s
	24m 18s

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