Let GSL build a shared library on Cygwin

Imported from <a href="http://trac.sagemath.org/sage_trac/ticket/14096">trac #14096</a>.

Spkg diff, for review only.

Reported by: jpflori

Component: cygwin

CC: kcrisman,, dimpase

Report Upstream: Workaround found; Bug reported upstream.

Authors: Jean-Pierre Flori

Dependencies: 

Work issues: 

Reviewers: Dmitrii Pasechnik

Merged in: sage-5.8.beta0

Attachments: <ol><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/14096/gsl-1.15.p1.diff">gsl-1.15.p1.diff: Spkg diff, for review only.</a> by jpflori at 20130212T19:13:41</li></ol>