fix gm on Mac, and Linux buildbot build, by adding zlib source to third_party

Mike- I understand that adding all this zlib code to third_party is, in your estimation, a step in the wrong direction.  But it will get gm working again on Mac (and should greenify the production buildbot as well).

I think the most important thing is to get those things working again; once that's done, I will find the cleanest way to do this (hopefully deleting libpng and zlib from third_party).

[reed1, 2011-06-15 16:03:25]

LGTM