Debian Stable (Buster) has GCC 8 and Ubuntu LTS (18.04) has GCC 7.

This will unlock a lot of C++17 features like `std::string_view`.

Move the existing systemd sections over there.

This reverts commit c84bae73.  A
configuration option is not necessary, because the PcmConvert
constructor knows already whether integer or floating point is needed.

On linux-rt, kernel IRQ threads are configured with priority=50, and
this change configures MPD somewhat below that priority, leaving some
room for other programs to be configured in between.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/643

Closes https://github.com/MusicPlayerDaemon/MPD/issues/42

This was missing in commit 0f488dce

Why would anybody want to keep the silence inserted by the codec?
Other plugins/codecs (such as Vorbis) have this hard-coded as well.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/616

Genres are not part of the SID format, so SID files are genreless. This
"default_genre" option may be used to assign a default genre to all SID
music, for example "SID", "C64", "Chiptune", etc.

This is useful in multiple mpd instances scenario, or multiple pulse outputs defined on the same mpd instance.
It is actually a more flexible way to route flows than the "sink" parameter, letting the PulseAudio routing do its job, but with the ability to isolate routing for each output.

If not specified, the role remains like it was before this commit, ie "music"

Time to move on, two years after 2017.

See https://github.com/MusicPlayerDaemon/MPD/issues/545