ffi: avoid loading 2 different zlib libraries (koreader#1912)

Avoid loading the system version instead of ours.

ffi/zipwriter.lua

@@ -14,7 +14,14 @@ local ffi = require "ffi"
 require "ffi/zlib_h"
 
 -- We only need to wrap 2 zlib functions to make a zip file
-local _zlib = ffi.load(ffi.os == "Windows" and "zlib1" or "z")
+local _zlib
+if ffi.os == "Windows" then
+    _zlib = ffi.load("libs/libz1.dll")
+elseif ffi.os == "OSX" then
+    _zlib = ffi.load("libs/libz.1.dylib")
+else
+    _zlib = ffi.load("libs/libz.so.1")
+end
 local function zlibCompress(data)
     local n = _zlib.compressBound(#data)
     local buf = ffi.new("uint8_t[?]", n)