Fix merge defect in ICU optional package declaration
Add more rules to simplify dealing with build features

* Introduce the notion of build feature attributes and add rules
  SetBuildFeatureAttribute and BuildFeatureAttribute to set respectively
  get a build feature attribute.
* Add rule ExtractBuildFeatureArchives to download and extract archives
  and set build feature attributes to extracted entries.
* Add rule UseBuildFeatureHeaders as a shorthand for getting a build
  feature attribute and adding it as a system header search directory.
Complete outsourcing of zlib

* 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.