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.

Drop some compatibility code.

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

Closes #392

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!

This is the version in Debian Jessie (oldstable).

av_register_all() was deprecated in
FFmpeg/FFmpeg@0694d8702421e7aff1340038559c438b61bb30dd

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

Use the duration of the stream we're actually decoding - not the
"global" attribute AVFormatContext::duration which may differ.

Needed for av_free().

Use the duration of the stream we're actually decoding - not the
"global" attribute AVFormatContext::duration which may differ.

Generate more detailed error messages.

Eliminate duplicate definition (in input plugin and decoder plugin).