The RWops stuff isn't a part of any standard PhysFS package (and given
that it explicitly wraps around SDL I'm not sure how you _would_ package
it). So we need to get the physfsrwops.h include in if
BUNDLE_DEPENDENCIES is off, otherwise this results in a compile-time
include-not-found failure.
Additionally, I've placed the PhysFS RWops stuff in their own extras/
folder, so none of the other PhysFS stuff gets included in a
-DBUNDLE_DEPENDENCIES=OFF build.
