systemd uses "rootprefix", not "prefix" for this value

https://github.com/systemd/systemd/blob/059b1b31ade7f1d716f56c5e6fd657ce22ce2032/src/core/systemd.pc.in#L23

See https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/

Fixes the build in nixpkgs

6d91b5c7 ("fix double promotions") changed
how LAME peak values are decoded, producing large incorrect values that
cause some MP3 files to play silently.

Restore the original decode from MAD fixed-point format to double and
document what it's doing.

Fixes #1823

With sphinx-build 5.0.0:
doc/user.rst:728: ERROR: Unexpected indentation.
doc/user.rst:731: ERROR: Unexpected indentation.

Let OnHeaders() check the status.

The status checking code was added by commit 4f021cbc in 2011,
but in 2008, commit a8e81326 enabled `CURLOPT_FAILONERROR`, which
means the status checking code never had any effect.

This allows `LoadExcludeListOrLog()` to hide boring "404 Not Found"
log messages via `IsFileNotFound()`.

The `assert(!quit)` can fail if the `EventThread` gets stopped before
it enters `EventLoop::Run()`. There is a similar problem with `alive`,
which gets reset by `EventThread::Stop()`.

If that happens, then `EventLoop::Run()` should return immediately
without handling any events.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1785

Without clearing all `in_playlist` flags, the songs will never be
revealed again if they were hidden once by a CUE sheet, not even after
the CUE sheet gets deleted or modified.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1784

Fixes GCC 10 error:

 error: cannot call member function `Foo` without object

Fixes crash inside AtScopeExit() in the WASAPI output plugin.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1759

This adds `MakeFfmpegError()` to the executable and fixes a linker
failure when `libavutil` is available, but `libavformat` and
`libavcodec` are not.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1786

Otherwise, building will generate these warnings:

systemd/system/meson.build:5: WARNING: Project targeting '>= 0.56.0' but tried to use feature deprecated since '0.56.0': Dependency.get_pkgconfig_variable. use Dependency.get_variable(pkgconfig : ...) instead
systemd/user/meson.build:5: WARNING: Project targeting '>= 0.56.0' but tried to use feature deprecated since '0.56.0': Dependency.get_pkgconfig_variable. use Dependency.get_variable(pkgconfig : ...) instead

libfmt 10 doesn't know how to format a StringView, and doesn't cast to
std::string_view anymore.  The StringView class has been removed from
MPD 0.24 completely, and this is a stable-branch-only workaround.

Closes https://github.com/MusicPlayerDaemon/MPD/pull/1814

This should prevent ffmpeg from taking priority over the gme plugin.
The ffmpeg plugin is more buggy than gme.
One of the prominent bugs of preferring ffmpeg over gme is that ffmpeg
cannot seek SAP files while gme can. This should prevent that from
happening.

libfmt version 10 has difficulties formatting a `StringBuffer`, and we
need to help it by explicitly invoking the `c_str()` method.

libfmt version 10 apparently doesn't know how to format a
`StringBuffer`, failing the MPD build.  Since we have a formatter
specialization for `AudioFormat`, let's use that - it's better and
easier to use.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1807

Fixes assertion failure if the ZIP file contains a path that begins
with a slash.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1793

Fixes hide_playlist_targets not working after server restart

Currently, `hide_playlists_targets` works by skipping songs with
`in_playlist` value set to true in
[`Directory::Walk`](https://github.com/MusicPlayerDaemon/MPD/blob/a57bcd02382947abbd961594cdf00302c0c7866a/src/db/plugins/simple/Directory.cxx#L237). But
`in_playlist` is not stored and only updated in
[`UpdateWalk::PurgeDanglingFromPlaylists`](https://github.com/MusicPlayerDaemon/MPD/blob/a57bcd02382947abbd961594cdf00302c0c7866a/src/db/update/Playlist.cxx#L139),
which will only be executed while updating DB.

This causes the problem that playlist target songs are correctly
hidden after database update, but will remain visible after mpd server
restarted. This pr solves the problem by storing `in_playlist` value
of songs into the `SimpleDatabase` file.