here is my fully working repo
just make sure to amend the .project file if you want to change the folder name and
classpath.libraries.location=${user.home}/Documents/Processing/libraries
to reflect the location of your processing sketchbook folder
and to change
classpath.local.location=${user.home}/Dcouments/processing-library-template-master/lib
to reflect the location where you want to place your library build files
Then just use ant to build.
As for the one provided by processing I could not get it to work.
Just bear in mind that only the final library file should be used and the core.jar should be deleted as it will cause conflicts either that or simply drag the final jar file to your code folder.