make a note about how to use Java 11

Author: benfry

build/build.xml

@@ -62,7 +62,7 @@
   </target>
 
   <!-- Require Java 11 everywhere. -->
-  <fail message="Unsupported Java version: ${java.version}. To build, make sure that Java 11 is installed.">
+  <fail message="Unsupported Java version: ${java.version}. To build, make sure that Java 11 is installed, and that JAVA_HOME points at the JDK that you want to use. For instance, on macOS, set it with:${line.separator}export JAVA_HOME=&quot;`/usr/libexec/java_home -v 11`&quot;">
     <condition>
       <not>
         <or>