Place the jar file in the ext/ directory of your gitblit installation.
Open your gitblt.properties file and add (or modify) this setting:
web.avatarClass = com.gitblit.ext.wavatar.WavatarGenerator
To build yourself, you must have the gitblit repository available. Add the following to the pom.xml if you don't have this repo in your maven settings already:
<repositories>
<repository>
<id>gitblit</id>
<url>http://gitblit.github.io/gitblit-maven/</url>
</repository>
</repositories>