Commit ebae25d1 added that #include, but forgot to wrap it in
"extern C", so the linker tried to look up C++ symbols, causing linker
failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1582

ffmpeg from current git master no longer exposes
av_malloc() nor av_free() through other included
headers. directly include libavutil/mem.h to fix
compilation with (as-yet-unreleased) ffmpeg.

This part of the AVIOContext API is not documented :-(

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1448

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.

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.