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.

This allows dropping a few compile-time version checks and we can use
pkg-config to detect the library.

This got lost 8 years ago in commit 87c88fcb

D'oh!

According to
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS_INIT.html
the _INIT variables should be set in the toolchain file.

Right now, zlib is only built for Windows, but we may eventually
changed that, so don't hard-code `win32/Makefile.gcc`.

Make sure all the gory details are visible in CI logs.

For better error logs on CI.