Fixes `-Wpedantic`.

Since Meson 0.51, there are special build options for "native:true"
builds, prefixed with "build.".  This change breaks cross builds
because `GenParseName.cxx` is no longer built with `-std=c++17`.

This patch adds defaults for "build.c_std" and "build.cpp_std".

Closes https://github.com/MusicPlayerDaemon/MPD/issues/890

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

We could exclude that feature if neither ALSA nor httpd are enabled,
but that's too complicated for this small debug program.

Fixes a build failure in `CommandLine.cxx`.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/905

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

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

The io_uring buffer is allocated as memlocked memory, as MPD needs to
be able to lock memory.

Before the advent of io_uring (commit dae8da70), this didn't
matter, because the `FileInputStream` never called this.  But
`UringInputStream` is derived from `AsyncInputStream`, and needs the
handler to signal completion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/898

For subprojects.