build: re-enable "other" loaders by default
2.42.11 disabled several loaders by default which are used or required by a variety of projects, including EOG, GKrellM, Nautilus, OpenSlide, and XSane. Some downstream distros have discovered this the hard way when breakage was reported, and there doesn't seem to have been an effort to discuss potential deprecations with other projects that use these loaders. Retain the ability to disable the loaders, since it seems useful, but re-enable them by default.