fix compilation error: rtable is only defined in OPENBSD
document compilation requirements for systemd support on Debian
make .gitignore more specific
tweak Makefiles to be noisier and use default rules and be more robust

- be less aggressive about suppressing printing of commands
- invoke subcommands once and save results
  - to make commands shorter
  - to make it easy to see the actual values
  - to be robust against changes like "git commit" during a build
- modularize CFLAGS vs CPPFLAGS
- use default rules where possible