Even Gentoo disables this plugin since before 0.9.8 release
of NetworkManager. Time to say goodbye.
If somebody happens to show up to maintain it, we may resurrect it
later.
If "$distro_plugins=ifnet" was set, configure.ac would use that
to autodetect --with-hostname-persist=gentoo. Replace that autodetect
part by checking for /etc/gentoo-release file.
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