configure: remove Cygwin

The Cygwin target is really compiling for native Win32 with -mno-cygwin.
Except, GCC 4.7.0 has finally removed the long deprecated -mno-cygwin
option, and that happened about five years ago.

Let it rest in peace.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>