Building using zlib on Windows
Danger Will Robinson - this is confusing and I'm not sure which procedure is right!
Using Gnuwin32's Binary
However, this doesn't give you a libzdll.a - I have yet to see if this works for building Abi. (It's needed for building libxml2 if you are building with zlib support)
Using Zlib.Net's Binary
This is what I've used for building Abi and plugins until now (22 dec 2007)
The procedure for installing zlib is a little bit more complicated than most, mainly because the Windows binary is intended for use in varied situations.
Copy the include directory to c:/mingw/ (overwriting/adding to the existing one)
Copy zlib1.dll to c:/mingw/bin/
Rename zdll.lib to libzdll.a and put it in c:/mingw/lib/
(If you are really adventurous, you can make a dependency package like for the others, with a bin/ directory containing zlib1.dll, the include/ directory it comes with, and a lib/ directory containing the renamed libzdll.a, for ease of re-installing your environment)