- Updated to upstream version 0.21.24.
- Enabled again iso9660 support.
- Switched to meson build system.

release v0.21.24

Our Windows build is built with `--disable-netrc`, and that makes
CURLOPT_NETRC fail, causing failures with all streams.  D'oh!

Closes https://github.com/MusicPlayerDaemon/MPD/issues/886

Closes https://github.com/MusicPlayerDaemon/MPD/issues/891

Closes https://github.com/MusicPlayerDaemon/MPD/issues/893

When the client wants to seek, but the decoder has already finished
decoding the current song, the player restarts the decoder with an
initial seek at the new position.  When this initial seek fails, MPD
pretends nothing has happened and plays this song from the start.

With this new flag, a restarted decoder marks the initial seek as
"essential" and fails the decoder if that seek fails.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/895

Needed for std::wmemchr under libcxx
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Fixes regression from commit 69f09648

This branch isn't yet ready for level 3 (`-Wpedantic`) due to several
C++ violations (e.g. variable length arrays).  These are already
cleaned up in the master branch (0.22).

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Closes https://github.com/MusicPlayerDaemon/MPD/issues/866

The WildMidi project added the pkg-config file in version 0.3.3, but
unfortunately, Debian still doesn't ship it 4 years later:

 https://bugs.debian.org/916631

However, for cross-compiling, the pkg-config file is very helpful.

Work around Meson bug https://github.com/mesonbuild/meson/issues/3672

Sync with the Android NDK build scripts.

This flag is used by the Android NDK build scripts as well, and this
fixes a build failure (assembler error) with FFmpeg and NDK r21.

The `db->close()` call was a `nullptr` dereference because the `db`
variable had already been moved.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/839

The command is used to configure the client's connection, and this
shouldn't require any permissions.  The client should be able to do
that before sending a password.

This reverts commit a5273d69.  It was
wrong and broke the MixRamp unit test.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/844