need help setting platform specific properties in ant

I have a build.properties file that was written by Windows developers. It looks like this:

jboss.home=D:/Development/jboss-4.0.4.GA

However, I'm building on linux and would rather have it look like this:

jboss.home=/opt/jboss-4.0.4.GA

Then I have a build.xml file that does this:

<property name="jboss.home" value="${jboss.home}"/>

So I have to edit the build.properties file before doing a build on linux.

What I'd like to do is something like this in build.properties:

jboss.home.windows=D:/Development/jboss-4.0.4.GA
jboss.home.unix=/opt/jboss-4.0.4.GA

Then, in the build.xml file do something like this:

<if> os=windows
<property name="jboss.home" value="${jboss.home.windows}"/>
</if>
<if> os=unix
<property name="jboss.home" value="${jboss.home.unix}"/>
</if>

....or something similar.

How do I do this?

Thanks.
.