They may be a couple of ways to solve this, but none are as straightforward as using the ant-contrib element. I'm not sure if this will get you what you need for your application, but you could try the following:
Using conditional targets. If you can replace your macrodef with a target to call, this may work for you. Note that this will set the property globally, so it might not work for your application.
<target name="default">
<condition property="platformIsProd">
<equals arg1="${platform}" arg2="prod" />
</condition>
<antcall target="do-buildstamp" />
</target>
<target name="do-buildstamp" if="platformIsProd">
<echo>doing prod stuff...</echo>
</target>
Handle the 'else' case. If you need to handle an alternate case, you'll need to provide a few targets...
<target name="default">
<property name="platform" value="prod" />
<antcall target="do-buildstamp" />
</target>
<target name="do-buildstamp">
<condition property="platformIsProd">
<equals arg1="${platform}" arg2="prod" />
</condition>
<antcall target="do-buildstamp-prod" />
<antcall target="do-buildstamp-other" />
</target>
<target name="do-buildstamp-prod" if="platformIsProd">
<echo>doing internal prod stuff...</echo>
</target>
<target name="do-buildstamp-other" unless="platformIsProd">
<echo>doing internal non-prod stuff...</echo>
</target>
Using an external build file. If you need to make multiple calls with different values for your property, you could isolate this in another build file within the same project. This creates a bit of a performance hit, but you would not need the additional library.
in build.xml:
<target name="default">
<ant antfile="buildstamp.xml" target="do-buildstamp" />
<ant antfile="buildstamp.xml" target="do-buildstamp">
<property name="platform" value="prod" />
</ant>
<ant antfile="buildstamp.xml" target="do-buildstamp">
<property name="platform" value="nonprod" />
</ant>
</target>
in buildstamp.xml:
<condition property="platformIsProd">
<equals arg1="${platform}" arg2="prod" />
</condition>
<target name="do-buildstamp">
<antcall target="do-buildstamp-prod" />
<antcall target="do-buildstamp-other" />
</target>
<target name="do-buildstamp-prod" if="platformIsProd">
<echo>doing external prod stuff...</echo>
</target>
<target name="do-buildstamp-other" unless="platformIsProd">
<echo>doing external non-prod stuff...</echo>
</target>
Add ant-contrib to your project. Of course, if you can add a file to your project, the easiest thing would be to just add the ant-contrib.jar file. You could put it under a "tools" folder and pull it in using a taskdef:
<taskdef resource="net/sf/antcontrib/antlib.xml" classpath="${basedir}/tools/ant-contrib.jar" />