av_frame_get_buffer() leaks memory if buffers were already allocated.

Fixes one of the memory leaks of https://github.com/MusicPlayerDaemon/MPD/issues/514

avio_read_partial() was added in libavformat 57.81.100, and we keep
compatibility with version 57.40 for now.  Fixes regression from
commit bfb7b011

Closes https://github.com/MusicPlayerDaemon/MPD/issues/511

Closes https://github.com/MusicPlayerDaemon/MPD/issues/510

.. and not "UNIX domain socket.  Be consistent about the naming.

MPD sometimes uses version numbers like "0.22~git" to mark unreleased
versions.  That makes the win32 resource compiler unhappy, because it
expects numbers only.

Since version 0.49.0 the Meson build system has native support for
finding and using the gcrypt library using the `dependency()` function.

`dependency()` has the advantage over `find_library()` as it queries the
required linker flags for proper linking with external libraries, e.g.
libgpg-error.

As the latest released version 1.8.4 of libgcrypt does not
provide a .pc file, using `libgcrypt-config` is the only way to query
the required linker flags.

Unfortunately, there is an issue when cross compiling mpd and the user does not
define `libgcrypt-config` in the cross file. If the user sets the qobuz feature
to `auto` and the target does not have libgcrypt installed, the Meson
build system will falsly assume libgcrypt is available for the target as
it uses the native `libgcrypt-config` on the host and pretend is has
found the library.

Therefore, we still rely on `find_library()` to workaround this buggy
behavior. This way, if qobuz feature detection is set to `auto`, the
feature is disabled in case there is no target libgcrypt available.

Fixes building mpd statically with the qobuz feature enabled. Otherwise
the build fails with undefined references because of the missing libgpg-error
dependency:

```
/sysroot/usr/lib/libgcrypt.a(libgcrypt_la-visibility.o): In function `gcry_strerror':
visibility.c:(.text+0x14): undefined reference to `gpg_strerror'
```

Meson 0.49.0 adds native support for `libgcrypt-config` which is
necessary for detecting libgcrypt dependencies, as the latest
version 1.8.4 of libgcrypt does not provide a .pc file.

release v0.21.6

Closes https://github.com/MusicPlayerDaemon/MPD/issues/465 and
https://github.com/MusicPlayerDaemon/MPD/pull/466

And disable libvorbis detection if Tremor was explicitly enabled.

This fixes a crash bug caused by libvorbis/Tremor ABI conflict caused
by commit 4f7d52db

Previously, MPD would skip the current song after attempting to seek
to its beginnig, because that was a seek to offset 0.  At offset 0,
MPD will see the BOS packet again, which results in throwing
StopDecoder in MPDOpusDecoder::OnOggEnd().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/470