Windows must use zlibstatic.

[xpol]

The zlib C library will have dll library named zlib.dll.
This rock will create a dll named zlib.dll.

With the same name, in runtime, the system will try to find zlib C library's functions in this rock's zlib.dll which will fail.

[brimworks]

I'm a little confused here... This rock should be creating a zlib.dll which is on your LUA_CPATH and NOT on your PATH, so there should not be a naming collision since windows dynamic libraries are loaded based on the PATH environment variable and not on the LUA_CPATH. Similarly, the lua runtime should be looking for C extensions using the LUA_CPATH environment variable and not the PATH environment variable.

I'm reluctant to force static linkage of this C extension since it is very likely that another C extension will dynamically load zlib (it is a very common library). In that situation AFAIK things break in unexpected ways if the two zlib versions are not the same since some symbols may come from the statically linked zlib and others may come from the dynamically linked version.

Can you verify that your PATH and LUA_CPATH environment variables have the appropriate setting?