Closes #477

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 function is sparsely documented and a look at the bluez-alsa
source code shows that implementations make undocumented assumptions
on the `struct pollfd` array parameter which can lead to strange
effects.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

This feature requires `libpcre`.

This call was missing, causing very high CPU usage when the ALSA
output plugin was used with dmix.

Closes #391

Fixes potential compiler error when zlib is installed in a
non-standard directory.

Fixes #414

Drop some compatibility code.

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

Fixes linker failure when building without FLAC support.

Closes #400

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

Closes #392

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!

GCC 6.3.0 emits bogus warnings like this:

 In file included from src/lib/dbus/UDisks2.cxx:22:0:
 src/lib/dbus/ReadIter.hxx: In instantiation of 'ODBus::ReadMessageIter::ForEachProperty(F&&)::<lambda(auto:2&&)> [with auto:2 = ODBus::ReadMessageIter; F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]':
 src/lib/dbus/ReadIter.hxx:102:6:   required from 'ODBus::ReadMessageIter::ForEachRecurse(int, F&&)::<lambda(auto:1&&)> [with auto:1 = ODBus::ReadMessageIter&; F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]'
 src/lib/dbus/ReadIter.hxx:92:5:   required from 'void ODBus::ReadMessageIter::ForEach(int, F&&) [with F = ODBus::ReadMessageIter::ForEachRecurse(int, F&&) [with F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]::<lambda(auto:1&&)>]'
 src/lib/dbus/ReadIter.hxx:101:3:   required from 'void ODBus::ReadMessageIter::ForEachRecurse(int, F&&) [with F = ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]::<lambda(auto:2&&)>]'
 src/lib/dbus/ReadIter.hxx:114:3:   required from 'void ODBus::ReadMessageIter::ForEachProperty(F&&) [with F = std::_Bind<void (*(std::reference_wrapper<UDisks2::Object>, std::_Placeholder<1>, std::_Placeholder<2>))(UDisks2::Object&, const char*, ODBus::ReadMessageIter&&) noexcept>]'
 src/lib/dbus/UDisks2.cxx:71:30:   required from here
 src/lib/dbus/ReadIter.hxx:114:53: error: declaration of 'ODBus::ReadMessageIter&& i' shadows a parameter [-Werror=shadow]
    ForEachRecurse(DBUS_TYPE_DICT_ENTRY, [&f](auto &&i){
                                                      ^
 src/lib/dbus/ReadIter.hxx:101:33: note: shadowed declaration is here
    ForEach(arg_type, [&f](auto &&i){
                                  ^

Since nfs_stat_async is deprecated.

This is the version in Debian Jessie (oldstable).

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330