warning: ‘includeantruntime’ was not set, defaulting to build.sysclasspath=last; set to false for repeatable builds

It is caused by a misfeature introduced in Ant 1.8

Include attribute includeantruntime and set it to false as shown below

Solution 1

 <javac .... includeantruntime="false" .../>  

Solution 2
In case if your project contains many build.xml files which one cannot, or prefers not to, edit
Use presetdef add these lines in top of build.xml file

 <presetdef name="javac">
    <javac includeantruntime="false" />

If your projects do actually need ant runtime libraries, you can either add them explicitly to your build files OR set includeantruntime=”true”

<javac destdir="out" includeantruntime="true">
  <src path="" />
  <src path="" />

Now all subsequent javac tasks will essentially inherit includeantruntime=”false”