Removed metakit and zlib from the configure/build process. The

Removed metakit and zlib from the configure/build process.  The
developer will need to build and install these separately if they don't
have packages already installed by their system.  See README.metakit and
README.zlib for more details.

As a convenience, pristine tarballs of the metakit and zlib source code
trees are included with the simgear distribution (and cvs.)