subr/build_checks.subr: don't build given --as-needed and no new commits to midipix_build since the last build.
    etc/build.usage, subr/build_args.subr: adds --as-needed.
    vars/build.vars:dist_target: adds dist_gitref.
    vars/dist_gitref.vars: print git-rev-parse(1) HEAD to ${PREFIX}/build.gitref.