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

Unlike pa_channel_map_init_auto(), pa_channel_map_init_extend() does
not fail if there is no valid mapping for the given channel count, but
instead maps additional "AUX" channels.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/493

The `tag_builder.empty()` check was wrong for the SubmitReplayGain()
call.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/497

Clarification for https://github.com/MusicPlayerDaemon/MPD/issues/505

The old implementation didn't make a lot of sense; the "!=" operator
was not actually the opposite of "==".

Closes https://github.com/MusicPlayerDaemon/MPD/issues/505

A few of those tests fail due to bugs.