OF-1149: Improve build speed
OF-1149: Improve build speed

* 'override' defined just once (and set to false)
* a plugin can be skipped by setting a property 'plugin.pluginname.skip'
  to true (eg: -Dplugin.broadcast.skip=true)
* plugins that are built set the property above, to prevent rebuilding
  during the same Ant invocation (speeds up parent/child plugin builds)

speedy01/Openfire,igniterealtime/Openfire,guusdk/Openfire,akrherz/Openfire,akrherz/Openfire,speedy01/Openfire,GregDThomas/Openfire,akrherz/Openfire,akrherz/Openfire,igniterealtime/Openfire,speedy01/Openfire,GregDThomas/Openfire,GregDThomas/Openfire,GregDThomas/Openfire,speedy01/Openfire,Gugli/Openfire,Gugli/Openfire,Gugli/Openfire,guusdk/Openfire,igniterealtime/Openfire,guusdk/Openfire,Gugli/Openfire,akrherz/Openfire,guusdk/Openfire,guusdk/Openfire,Gugli/Openfire,speedy01/Openfire,GregDThomas/Openfire,igniterealtime/Openfire,igniterealtime/Openfire