Since we switched from autotools to Meson in commit
94592c1406, 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.
From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else. DetachedSong is easier
to use, but Song has lower overhead.
