Recognize cygwin


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13976 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/autoconf/configure.ac b/autoconf/configure.ac
index 0e9e318..64c5b62 100644
--- a/autoconf/configure.ac
+++ b/autoconf/configure.ac
@@ -103,6 +103,9 @@
 	           AC_SUBST(LLVMGCCDIR,[/home/vadve/lattner/local/sparc/llvm-gcc/])
 			 fi
 	         ;;
+	*-*-cygwin*)
+             AC_SUBST(OS,[Cygwin])
+	         ;;
 
 	*-*-darwin*)
              AC_SUBST(OS,[Darwin])