Avoid using -Wl,-z,defs on Cygwin.

Prior to CMAKE 2.8.4 that was covered by the WIN32 conditional but
from 2.8.4 CMAKE no longer defined WIN32 when running under Cygwin
and it needs its own test.

Patch by Martell Malone!

http://reviews.llvm.org/D11347

llvm-svn: 242993