COMP: Use internally consistent zlib

The zlib libraries are used by many differnt packages
includeing ITKv4, LibArchive, VTK, python, and teem.

This patchset builds a single version of zlib
that is mangled with a slicer namespace, and uses
that one library for all zlib operation throught the
"USE_SYSTEM_ZLIB" options.

The mangling for vtk_zlib is disabled for teem because
it is not needed when using the External_zlib build.