- add libbz2-dev as new build-dependency

  - add libbz2-dev as new build-dependency
  - remove the libz-dev alternative from zlib1g-dev build-dependency
  - do the same for bz2 builtin if available
* apt-pkg/contrib/fileutl.cc:
  - use libz2 library for (de)compression instead of the bzip2 binary as
    the first is a dependency of dpkg and the later just priority:optional
    so we gain 'easier' access to bz2-compressed Translation files this way