Fixes `-Wpedantic`.

Commit
https://github.com/mesonbuild/meson/commit/08224dafcba1b694fb624553e7d84deb565aae22
changed Meson to require BOOST_ROOT for cross builds.

A bug report (https://github.com/MusicPlayerDaemon/MPD/issues/912)
suggests that on Linux, reading on `cifs` files may rarely return 0 (=
end of file) before the end of the file has really been reached.  But
that's just a theory which I need to validate, so this runtime check
shall catch this condition before the assertion in
DecoderBridge::Read() crashes MPD.  Let's see.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/912

Closes https://github.com/MusicPlayerDaemon/MPD/issues/910

An assertion failure in UpdateQueuedSong() could trigger because the
`prev` parameter is always `nullptr`, but `queued` may be set.  And in
fact, calling UpdateQueuedSong() is only necessary when the queued
song was edited, to re-queue it with the new range.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/901

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.