(partly) fixing `make uninstall` for ruby:
(partly) fixing `make uninstall` for ruby:

- both "install-exec-local-*" & "install-data-local-*"
  appear to be matched by "uninstall-local-*" in our case;
  hence, also for ruby/gem we use ""uninstall-local-*"
  instead of "uninstall-exec-local-*"

- the actual `gem uninstall ...` call still fails with
  "ERROR:  While executing gem ... (Gem::InstallError)"
  I have no idea (yet??) why ...

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