A plug-in in Eclipse can be built with an Ant build script. To create a build script for your AspectJ plug-in right click on the plugin.xml file in the package explorer then select PDE Tools > Create Ant Build File with AspectJ Support. This will generate a build.xml file that can be used to build your plug-in.
Creating a new AspectJ plug-in project
Converting a plug-in project to an AspectJ project
Creating a new Ant build file