This is a heavily modified and shrinked version of zlib 1.2.3

- Removed comments from zlib.h
- Removed gzip/zip archive I/O
- Removed infback.c
- Added Assimp #idefs to exclude it if not needed
- Disabled debug macros in zutil.h

Assimp itself does not use the compression part yet, so
it needn't be compiled (trees.c, deflate.c, compress.c).
Currently these units are just used by assimp_cmd.
