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.

Closes #389

Fixes `-Werror=odr`

This code (added 7 years ago with commit b233c145) has been obsoleted
by the SongLoader class (added 3 years ago).

Mapped to "=" in the MPD protocol.  This is the first operator,
initially supporting value matches in the MPD protocol.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Don't depend on the "simple" database plugin.  This fixes an assertion
failure / crash and allows using stickers with other plugins.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.