Make it easier, during release, to verify that JDK9+ is being used

ReleaseInstructions

@@ -43,6 +43,10 @@ Note: This document was adapted from the one created in the context of
     the right year and commit and push the change upstream, before
     starting any release process.
 
+    There are certain features that require JDK 9+ version to be used
+    while releasing Ant 1.10.x from master branch. Make sure JAVA_HOME
+    points to a JDK installation which is minimally Java 9.
+
 3.  We don't want tags for failed votes to end up on our branches so
     the release is first created from a detached head.
 
@@ -74,6 +78,14 @@ Note: This document was adapted from the one created in the context of
 6.  Next bootstrap, build and run the tests.
 
     $ ./bootstrap.sh
+
+    Make sure the log messages doesn't contain the message:
+
+    "Java 9+ features won't be available in the distribution"
+
+    If it does, then it's a sign that JAVA_HOME isn't pointing to a JDK 9+ version.
+    Fix JAVA_HOME to point to a valid JDK 9+ version and redo the release steps.
+
     $ ./build.sh
     $ ./build.sh test
     # if you've got maven installed

build.xml

@@ -952,7 +952,7 @@
          Create the essential distribution that can run Apache Ant
        ===================================================================
   -->
-  <target name="dist-lite" depends="jars,test-jar"
+  <target name="dist-lite" depends="jars,test-jar,-ant-dist-warn-jdk9+"
           description="--> creates a minimum distribution to run Apache Ant">
 
     <mkdir dir="${dist.dir}"/>
@@ -2008,6 +2008,10 @@ ${antunit.reports}
       <echo>AntUnit tests must be run with ${bootstrap.dir} (or ${dist.dir}), not ${ant.home}. Try './build.sh antunit-tests' for example.</echo>
   </target>
 
+  <target name="-ant-dist-warn-jdk9+" unless="jdk9+">
+      <echo>Java 9+ features won't be available in the distribution</echo>
+  </target>
+
   <target name="antunit-tests" depends="build,test-init,-antunit-warn-location" if="run.antunit"
           description="--> run the antunit tests">
     <condition property="antunit.includes" value="${antunit.testcase}"