get-deps.sh: Use a stable URL for zlib so we don't break when zlib releases.

zlib only maintains the most current version at zlib.net, so using that URL
breaks everytime they put a new version out.  Archived versions are maintained
as part of the libpng setup at sourceforge, but sourceforge makes it somewhat
annoying to directly download files.

* get-deps.sh
  (HTTP_FETCH): Pass -L option to curl so it'll follow redirects.
  (get_zlib): Use a URL on sourceforge that provides stable and direct
    downloads.  However, it results in the file being named download by
    the various tools we use to fetch files.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552957 13f79535-47bb-0310-9956-ffa450edef68