incompatible zlib libraries

[daurnimator]

Creating this to track #28 (comment) independantly

Relevant: brimworks/lua-zlib#24

lzlib isn't the only lua library that claims 'zlib' for it's require string.
Can we feature detect which zlib library gets loaded?

Both 'lua-zlib' and 'lzlib' have 'zlib' for their require string. This causes issues with luarock's zip file handling
e.g. If lua-zlib is installed, packing a rock will fail

[daurnimator]

If anyone is playing along at home, I wrote this module to abstract over the two: https://github.com/daurnimator/lua-http/blob/master/http/zlib.lua