build.gradle

String getJavaHome(String version)
{
    def stdout = new ByteArrayOutputStream()
    exec {
        commandLine "/usr/libexec/java_home", "-v", version
        standardOutput = stdout;
    }
    return stdout.toString().trim()
}

retrolambda {
    jdk System.getenv("JAVA8_HOME") ?: getJavaHome("1.8")
    oldJdk System.getenv("JAVA7_HOME") ?: getJavaHome("1.7")
    // ...
}