Fixes crash bug.

This should translate to ACK_ERROR_ARG instead of ACK_ERROR_UNKNOWN.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1265

This way, we know whether we're explicitly "paused" or "not yet
activated because the ring buffer hasn't been filled yet".

As per Meson 0.56.0:
 https://mesonbuild.com/Machine-files.html#properties

Fixes the build with libexpat from Meson wrap.

Not necessary anymore since Meson 0.50.0:
 https://mesonbuild.com/Release-notes-for-0-50-0.html#libdir-defaults-to-lib-when-cross-compiling

Requires Meson 0.56.0:
 https://mesonbuild.com/Release-notes-for-0-56-0.html#per-subproject-warning_level-option

Requires Meson 0.54.0:
 https://mesonbuild.com/Release-notes-for-0-54-0.html#per-subproject-default_library-and-werror-options

I want to use per-subproject warning_level (0.56.0), default_library
(0.54.0).

Necessary since commit 6acf81d5

Let cmake know that we're cross-crompiling.

Prepare to add some code around the _build() call.

Eliminate conflict with other Project classes.

Allow DeferredEvents to call EventLoop::Break().

emplace already calls std::pair. No need for it again.

No need to emplace when calling std::make_shared.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

make_pair is an old C++98 function that can be replaced by modern
shorter constructs.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

NB: Check the sysconfdir setting to determine where mpd will look for mpd.conf; if you expect mpd to look for /etc/mpd.conf the sysconfdir must be '/etc' (i.e., not 'etc' which will result in mpd looking for /usr/local/etc/mpd.conf):
 
.. code-block:: none
 
 meson configure output/release |grep sysconfdir

If this is not /etc (or another path you wish to specify):

.. code-block:: none

 $ meson configure output/release -Dsysconfdir='/etc' ; meson configure output/release |grep syscon
  sysconfdir              /etc                                               Sysconf data directory

This eliminates all but one DeleteLocalRef() call.