Using If condition in Ant javac patternset

I would like to exclude some packages to compile, if a variable was true or false. Below is my ant configuration, but I get error:

<javac srcdir="src;src-generated" destdir="classes" debug="true" encoding="utf-8">

        <if>
            <equals arg1="${compile.csv}" arg2="false"/> 
            <then>
  <exclude name="${csv.package}"/>
      <exclude name="${excel.package}"/>
            </then>
            </if>

javac doesn't support the nested "if" element.

Thank you in advance for your answers.