zlib: clean up zlib.gyp, don't build minizip

As exposed by #273, the bundled zlib builds an optional extension called 'minizip' that provides filesystem-like APIs for working with zip files. Node doesn't use it, so let's not build it either.

This patch works on windows but I haven't checked other platforms yet. (\o CI). What definitely needs to be reviewed is what happens if use_system_zlib is defined; with this patch zlib still gets built but has no sources at all.

R=@bnoordhuis or @shigeki