Fixed Windows build

I did the following things:

 * Added libz as a library that needs to be linked. (Failed without)

 * Renamed .def that weren't Windows .def files to .cdef, to avoid
   possible disaster.

 * Made it use "oil.pyd" instead of "oil.so" on nt systems.

 * Added -Wl,--export-all-symbols to the oil build as it would only
   export the module init function on Windows without this option.

Now builds with mingw-w64 for me.