Internally at <a href="http://www.acquia.com>Acquia we had the need to add a custom plugin to our Solr 3.4 Build. This was already developed for Solr 1.4 but since Solr 3.x is using a very different approach while building and compiling its contrib folder this became a challenge.
I'll warn you because this is probably a very uncommon use-case for 90% of the users.
Normally the process of building Solr goes like this :
Get the source code from the subversion repository.