Add info about JAVA_HOME possibly being set but not exported

PR:	14181


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@273954 13f79535-47bb-0310-9956-ffa450edef68

src/script/ant

@@ -151,7 +151,7 @@ if [ -n "$JAVA_HOME" ] ; then
     fi
   fi
 else
-  echo "Warning: JAVA_HOME environment variable is not set."
+  echo "Warning: JAVA_HOME environment variable is not set (or not exported)."
   echo "  If build fails because sun.* classes could not be found"
   echo "  you will need to set the JAVA_HOME environment variable"
   echo "  to the installation directory of java."