To put all together;
<property environment="env"/>
<condition property="user.username.dir" value="user/${env.USERNAME}" else="
user/${env.USER}">
<os family="windows" />
</condition>
<echo message="user.username.dir=${user.username.dir}" />
<import file="${user.username.dir}/build.local.xml" optional="true"/>
I think env.USER will return username on mac systems as well.
