Enable bootstrap to be executed multiple times without prompting on

Windows.  Does not appear to be a problem on Unix.
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git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@268768 13f79535-47bb-0310-9956-ffa450edef68

bootstrap.bat

@@ -20,6 +20,8 @@ echo ... Bootstrapping Ant Distribution
 
 if     "%OS%" == "Windows_NT" if exist bootstrap rmdir/s/q bootstrap
 if not "%OS%" == "Windows_NT" if exist bootstrap deltree/y bootstrap
+if     "%OS%" == "Windows_NT" if exist build\classes rmdir/s/q build\classes
+if not "%OS%" == "Windows_NT" if exist build\classes deltree/y build\classes
 
 SET LOCALCLASSPATH=lib\crimson.jar;lib\jaxp.jar;lib\optional\junit.jar