Upgrade zlib to 1.2.6

Imported from <a href="http://trac.sagemath.org/sage_trac/ticket/12800">trac #12800</a>.

Diff for the zlib spkg. For review only

Reported by: jdemeyer

Component: packages

CC: mjo

Report Upstream: N/A

Authors: Jeroen Demeyer

Dependencies: 

Work issues: 

Reviewers: Michael Orlitzky, Julien Puydt, Leif Leonhardy

Merged in: sage-5.0.beta14

Attachments: <ol><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/12800/zlib-1.2.6.reviewer-ll.patch">zlib-1.2.6.reviewer-ll.patch: Reviewer patch.  Apply to Jeroen's spkg.</a> by leif at 20120411T12:08:47</li><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/12800/zlib-1.2.6.diff">zlib-1.2.6.diff: Diff for the zlib spkg. For review only</a> by jdemeyer at 20120411T12:54:55</li></ol>