update: update zlib to version 1.2.8 ( see https://github.com/assimp/assimp/issues/134 )

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>
bugfix : add cmakein file for zlib.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>
bugfix: add missing zconf file for cmake.

Signed-off-by: Kim Kulling <kim.kulling@googlemail.com>