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.

These were disabled by accident.

Use some odd number that will expose problems with remaining samples
after optimized vector operations.

Replacing GLib macros.

GLib 2.16 was released more than 4 years ago.  Let's remove some cruft
from the glib_compat.h header, and avoid new cruft to it.

Eliminate support for reverse endian samples from the MPD core.  This
moves a lot of complexity to the plugins that really need it (only
ALSA and CDIO currently).

Didn't compile on GLib < 2.16.