upgrading zlib?

[Tom Tromey]

PR 14856, upgrading our zlib, has been around for a while.  I took a
look at this, and the new zlib pretty much just dropped in.  I had to
tweak the Makefile.am (file names changed), but configure remained
untouched.

I think it is friendlier for us to ship an up-to-date release of zlib.
Plus, it promises a performance improvement.  Finally, I think it
would be pretty safe to import this now.  I have been running it here
for a few days without any trouble.

However, I realize it is stage 3 and I wanted to give people, and in
particular Mark, a chance to veto this due to bad timing or other
concerns.  Comments?

Tom