Support amsrefs

[rekka]

This commit changes the condition that decides whether bibtex is run to
finding \bibdata in the aux file (was \citation and \bibcite).
Since bibtex exits with an error if it does not find \bibdata anyway,
this avoids running bibtex unecessarily when for instance amsrefs is
used to process the bibliography.

I also replaced find(...).count() > 0 by find(...).next().is_some(): no need to go through the whole file, stop at the first match.

[pkgw]

I will confess that I don't fully understand the meaning of the control sequences that you're talking about here, but you sound like you know what you're doing :-) Thanks for the patch!

[rekka]

Thanks for merging!

I have to admit that this info is based on some experimentation and on how latex defines the \bibliography command in latex.ltx:

\def\bibliography#1{%
  \if@filesw
    \immediate\write\@auxout{\string\bibdata{#1}}%
  \fi
  \@input@{\jobname.bbl}}