<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<project name="test" basedir="." default="main">

  <property name="lib.dir.gtk" value="/opt/java_libs/gtk64" />
  <property name="lib.dir.win" value="/opt/java_libs/win" />

  <condition property="lib.dir" value="${lib.dir.gtk}" else="${lib.dir.win}">
    <or>
      <and>
        <isset property="${build.os}" />
        <equals arg1="${build.os}" arg2="gtk" />
      </and>
      <and>
        <not><isset property="${build.os}" /></not>
        <os name="Linux" />
      </and>
    </or>
  </condition>

  <target name="main">
    <echo message="${build.os}" />
    <echo message="${lib.dir}" />
  </target>

</project>