- (dtucker) [configure.ac] On Cygwin the getopt variables (like optargs,
   optind) are defined in getopt.h already.  Unfortunately they are defined as
   "declspec(dllimport)" for historical reasons, because the GNU linker didn't
   allow auto-import on PE/COFF targets way back when.  The problem is the
   dllexport attributes collide with the definitions in the various source
   files in OpenSSH, which obviousy define the variables without
   declspec(dllimport).  The least intrusive way to get rid of these warnings
   is to disable warnings for GCC compiler attributes when building on Cygwin.
   Patch from vinschen at redhat.com.