Add ZLIB subsetting script and update snapshots to latest (1.2.8)

This creates a new make target:

    make zlib

When you run that, it will execute the script **%src/tools/make-zlib.r**.  This will process the Zlib repository to produce two snapshot files that Rebol requires: **%src/include/sys-zlib.h** and **%src/core/u-zlib.c**.

*(Note: Until HTTPS support is in mainline, you will have to download or clone the Zlib sources locally and edit the path in the script to point to wherever you put it.)*

This snapshotting process would be run periodically after new releases of Zlib.  The subsetting script would need to be updated in case of significant changes to the library's structure.

This appears to resolve [CC #2040](http://curecode.org/rebol3/ticket.rsp?id=2040), as with the change I can now successfully save PNG files on Linux.