Make it less generic, to avoid clashes.

Closes GH-479.

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

Closes #391

ensure that valid mixer values are set also when the ALSA driver
does not report a valid dB range ('set_raw' fallback)

correct a bug in which volume is assumed to lie in [0..100]
instead of [0..1]

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!

Due to rounding errors, a slightly negative value can be passed to
set_normalized_volume(), which will make the log10() call fail.
Actually, volume 0 is already failing because log10(0) is illegal.  So
let's fix this by implementing two corner cases: <=0 and >=100.

Closes #212

This alleviates a problem in which 'volume +1' cannot be undo by
'volume -1' when using alsa hw mixer.

Closes #104

Fix breakage from commit bea5681f

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

This commit is similar to 788e3b31,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Will move attributes from struct AudioOutput that are specific to the
OutputThread.  The new struct AudioOutputControl is a holder for the
AudioOutput pointer.

This prepares for making the output list more dynamic, to allow moving
outputs to between partitions.

Some missing bits when converted to C++ exceptions...

Yet another potential crash bug fix.