Recognize cygwin


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13976 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/configure b/configure
index bc51db0..139648d 100755
--- a/configure
+++ b/configure
@@ -1869,6 +1869,10 @@
              OS=Darwin
 
 	         ;;
+	*-*-cygwin*)
+             OS=Cygwin
+
+	         ;;
 
 	*)       OS=Unknown