* Also make use of new build feature rules.
* Since the hacky long_jump_buffer field has been removed from the
jpeg_error_mgr struct in the new package, the structure is now
wrapped in the JPEGTranslator code to achieve the same behavior.
We have to use actual targets that cause the respective download and
extract the packages. Otherwise the build fails when the packages
haven't been extracted yet.
* Add optional packages Zlib and Zlib-devel.
* Simplify the build feature section for zlib and also extract the
source package.
* Replace all remaining references to the zlib instance in the tree and
remove it.
