message: Use giflib mirror on github
diff: url = https://github.com/madler/zlib.git
  [submodule "third_party/giflib/src"]
  path = third_party/giflib/src
  - url = https://git.code.sf.net/p/giflib/code
  + url = https://github.com/We-Amp/giflib-mirror.git
  