In mathgen.pl::generate_bibtex, the compute_re can be moved out of the loop

`compute_re` can be moved out of the loop by setting `$rules->{"CITE_LABEL_GIVEN"}` to 1 before the loop, recomputing re, then inside the loop setting the hashtable entry to `[$clabel]`. 

This has been tested in a local build to not introduce regressions and the issue exists mostly for the sake of attaching to a pull request.