#100 Wrong warnings if multiple bibtex files used

[Thorsten Schaefer]

The partial building of bibtex doesn't work correctly.
If you have the following bibtex file everything is fine:

@STRING{Elsevier_J_SS = "J. of Systems \& Software"}
@STRING{Pub_Elsevier = "Elsevier Science Inc."}

@article{vanGurp02,
author = {Jilles van Gurp and Jan Bosch},
title = {Design erosion: problems and causes},
journal = Elsevier_J_SS,
volume = {61},
number = {2},
year = {2002},
issn = {0164-1212},
pages = {105--119},
publisher = Pub_Elsevier,
}

If you move the string definitions (the first two
lines) into a separate file and include both bibtex
files into your document, it should work the same. The
produced DVI/PDF is fine, but you'll get two warnings
that the strings Elsevier_J_SS and Pub_Elsevier are
undefined.

I think one needs to parse all bibtex files used in the
document's bibliography together.