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.

Apparently on hurd-i386 __GLIBC__ is defined, but the pthread
implementation is special and cannot be used with constexpr. Hence
exclude __gnu_hurd__.

release v0.20.23