SDL2: Keep .a files on `dontDisableStatic`; don't move them to $dev; prune .la.

Most other packages don't move `.a` files to "$dev", and that is because
it makes the pkg-config `.pc` file wrong (the `libdir` is the non-dev one).

Keeping them in the main output makes static linking of SDL2 work.

See added comment about pruning of `.la` files.