Build script tweak

build.gradle

@@ -2,7 +2,7 @@ plugins {
   id 'java'
   id 'maven'
   id 'groovy'
-  id 'org.jastadd' version '1.13.1'
+  id 'org.jastadd' version '1.13.3'
 }
 
 if (!file('extendj/jastadd_modules').exists()) {
@@ -19,6 +19,8 @@ sourceSets {
     java {
       srcDir 'extendj/src/frontend'
       srcDir 'extendj/java8/src'
+      // This exclude rule needed if the submodule is built.
+      exclude 'gen/**' // Exclude from extendj/java8/src
     }
   }
 }