6915152: langtools build failures with import.jdk on Windows

Reviewed-by: ohair

make/build.xml

@@ -107,14 +107,21 @@
         ignoresystemclasses="true"
         classpath="${import.jdk}/jre/lib/rt.jar" classname="java.nio.file.Path"/>
 
+    <!-- Set the default value of the sourcepath used for javac. -->
     <condition property="javac.sourcepath" value="${build.genstubs.dir}" else="">
         <isset property="import.jdk.src.dir"/>
     </condition>
 
+    <!-- Set the default value of the classpath used for javac. -->
     <property name="javac.classpath" value=""/>
 
+    <!-- Set the default bootclasspath option used for javac. 
+	Note that different variants of the option are used, meaning we can't just 
+	define the value for the option.
+	Note the explicit use of the standard property ${path.separator} in the following.
+	This is because Ant is not clever enough to handle direct use of : or ; -->
     <condition property="javac.bootclasspath.opt"
-            value="-Xbootclasspath:${build.classes.dir}:${import.jdk.jar}"
+            value="-Xbootclasspath:${build.classes.dir}${path.separator}${import.jdk.jar}"
             else="-Xbootclasspath/p:${build.classes.dir}">
         <isset property="import.jdk.jar"/>
     </condition>