ABS is being considered for deprecation, so you should move to an alternative approach anyway.
The replacement for ABS is asp, a git based alternative for building from source.
For your required package, you just init the git repository (asp export glibc), and then whenever you need to update you can switch into the directory and run asp update glibc.
Because it is just a git repo, you can further simplify your workflow by creating your own branch with any patches etc., and then rebasing and running makepkg -i in that branch for your build and install.
asp is developed by Dave Reisner, one of the Arch developers, so will likely make it into the supported repositories once ABS is officially abandoned.