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

_GLIBCXX_USE_C99_MATH_TR1 is the correct one.

_GLIBCXX_USE_C99_MATH is always defined.

libnfs is compiled with `-D_FILE_OFFSET_BITS=64`, but Meson decides
not to enable this mode.  We could force this mode, but then again,
these days, nobody should be using 32-bit Windows ... so this is a
kludge only for debugging with 32-bit WINE.

Fixes Path::IsNull() checks on Windows.

This fixes a freeze bug in the NFS input/storage plugins: when libnfs
auto-reconnets after a failure, it installs the new socket on the same
file descriptor number.  MPD's attempt to unregister the old socket by
calling SocketMonitor::Steal() from NfsConnection::ScheduleSocket()
fails because the new/old socket number is not registered in epoll, so
epoll_ctl() returns ENOENT.  The problem is that it left
`scheduled_flags`, and so subsequent Schedule() calls will use
`EPOLL_CTL_MOD`, which will fail again and again.  Instead, we need to
use `EPOLL_CTL_ADD` to register the new socket.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/806

Closes https://github.com/MusicPlayerDaemon/MPD/issues/756