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.

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.

Didn't compile on GLib < 2.16.

This is easier and more efficient to loop on, because only two
variables get modified (src and dest).