Re-add build support for the fixed-point Vorbis (Tremor) decoder, which
was dropped when switching from Autotools to Meson.

Note, that it is not possible to build both, the Vorbis and the Tremor
decoder.

Closes: #405

libwrap is an obscure artefact from a past long ago, when source IP
address meant something.

And its API is "interesting"; it requires the application to expose
two global variables `allow_severity` and `deny_severity`.  This led
to bug #437.  I don't want to declare those variables; instead, I'd
like to remove libwrap support.

Closes #437

This feature requires `libpcre`.

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.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!