How to install Javassist to an Ant project

I'm trying to import javassist in one of my Ant projects, however, I'm finding it hard to be able to import the Javassist jars into my project. Here is my build.xml:

<target name="compile">
        <javac srcdir="src" destdir="classes" includeantruntime="false"/>
        <jar destfile="./KeyConstructors.jar" basedir="classes">
            <zipgroupfileset dir = "./lib" includes = "*.jar"/>
            <manifest>
                <attribute name="Main-Class" value="ist.meic.pa.KeyConstructor" />
            </manifest>
        </jar>
</target>

I have a "lib" folder in the root of the project, and inside I have a zipfile with Javassist downloaded from the official website. I tried placing the jar files on the lib folder, but nothing, I'm not able to import javassist from my project. Any ideas on what I'm doing wrong?

Thanks