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

This flag is output-only.

These flags are output-only.

These flags are output-only.  Using them here is misleading.

The object's state is `IDLE` when OnNfsCallback() gets invoked, so
let's use the start of the method to reset the `state` field.

Fixes assertion failure if the callback fails.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/832

Closes https://github.com/MusicPlayerDaemon/MPD/issues/833

Using CompareStringEx() and FindNLSStringEx().

Implements a missing piece for
https://github.com/MusicPlayerDaemon/MPD/issues/820