Some Boost 1.67 packages apparently have a workaround for the bug, so
let them build MPD.

Without the Vorbis decoder, `libvorbis` is never detected, leading to
linker failures when attempting to build the Vorbis encoder.

Closes #399

Meson 0.47.1 suffers from a bug which breaks linking the MPD
executable because the `-lpthread` flag is not propagated from our
`thread.a`.

See https://github.com/mesonbuild/meson/pull/3895

Closes #403

Fixes linker failure when building without FLAC support.

Closes #400

`HOST_NAME_MAX` is not a portable macro; it is undefined on some
systems.

Closes #402

Fixes #404

Fixes one part of #404

Closes #397

Closes #396

Thanks to C++14, we can declare and fill variables inside `constexpr`
functions.  This means me can stop make assumptions on the `struct`
layouts without losing `constexpr`.

Closes #393

`__linux` is the deprecated non-standard macros which appears to be
not present at all on PowerPC.

Closes #398

Apparently, Meson propagates the linker flags but not the compiler
flags from a `static_library`'s dependencies list.

Closes #392

std::regex_replace() is heavily bloated and overkill for this feature.

Closes #389

expand %h to the system hostname and default to a zeroconf string that
contains %h

Fixes writing to Context.getCacheDir().

All new hardware is arm64 anyway.

Bugs in libroar which broke the MPD build have been annoying me for
quite some time, and the newest bug has now hit my main build machine:

 https://github.com/MusicPlayerDaemon/MPD/issues/377

Problem is the usage of the typedef `_IO_off64_t` in libroar's
`vio_stdio.h`:

 int roar_vio_to_stdio_lseek (void *__cookie, _IO_off64_t *__pos, int __w);

This `_IO_off64_t` is an internal implementation detail of glibc and
was removed in version 2.28.  Nobody must ever use it.  Why the ****
did the RoarAudio developers use it?  Not using internal typedefs
isn't exactly rocket science.

This annoys me enough to finally remove the plugin.  Anyway, I've
never heard of anybody using RoarAudio, so my best guess is that
nobody will notice.

This keeps one central place for the XDG switch.  Specifically, this
disables the XDG listener on Apple, where XDG is not supported.