Simplify a couple of lines of code
Use > instead of >= when comparing file times for bibfiles

When LaTeX runs really quickly, the filesystem's mtime is not always
precise enough for it to have changed by the next time we call
run_bibtex
Assume references may have changed if aux file has changed

If you change a label and create an error at the same time, LaTeX will
generate an incomplete aux file.  For some (complex?) projects, LaTeX
then cannot tell that the references have changed when the error is
fixed.