Use -html4 for javadoc generation

javadoc in our classes don't yet comply to html5 semantics. When
generating javadocs for our project, using JDK 11, we run into
build failures since JDK 11 javadoc tool defaults to html5 output
and is strict if the classes don't comply to the semantics.

The commit here explicitly sets the output to html4 to avoid such
failures, till we fix our javadoc to comply with html5

build.xml

@@ -1503,6 +1503,14 @@
         <patternset refid="useful.tests"/>
       </srcfiles>
     </uptodate>
+    <!-- Javadoc on classes in Ant project, don't (yet) comply to HTML5
+    semantics. javadoc tool in JDK 9+ allows specifying the HTML version
+    to use for the output generation. We intentionally set it to -html4
+    to prevent javadoc tool failures (in JDK 11, which is strict) when
+    running against our code -->
+    <condition property="javadoc.output.htmlversion" value="-html4" else="">
+      <isset property="jdk9+"/>
+    </condition>
   </target>
 
   <target name="javadocs" depends="check-javadoc"
@@ -1535,6 +1543,7 @@
       <group title="Optional Types" packages="org.apache.tools.ant.types.optional*"/>
       <group title="Ant Utilities" packages="org.apache.tools.ant.util*"/>
       <classpath refid="tests-classpath"/>
+      <arg line="${javadoc.output.htmlversion}"/>
     </javadoc>
   </target>