The enclosed patch removes the inclusion of picins.sty by docs/titlepag.tex due to the following reasons:
Picins.sty is not provided by standard LaTeX distributions. Building gnuplot.pdf forces the user to search for the package on the net.
The package picins.sty is not necessary as no figures are not yet embedded by gnuplot.tex.
Picins.sty is a rather old package (1992) which is not maintained any longer. Furthermore, its documentation is only available in German.
The embedment of figures might also be done by using the functionality of the package graphicx.
Patch for docs/titlepag.tex
This package really _is_ required for inserting figures, so removing it breaks the target
make pdffigures
It seems strange that your latex installation is missing this package, but that is easy enough to fix. English language documentation for picins is available through CTAN (the usual resource for TeX packages), as is picins.sty itself.
http://mirrors.ibiblio.org/pub/mirrors/CTAN/macros/latex209/contrib/picins/picins.txt
Perhaps there is a way to make the \usepackage statement depend on the make target? That would be a more acceptable patch.
Or if you can find an example of another way to embed figures with textwrap, that would also be fine. But so far as I have been able to determine, there is no suitable replacement for picins.sty. I have tried several alternative packages that people have suggested to me for figure inclusion, but none of them produced acceptable results in my hands. Perhaps I failed to understand their proper use; that's why an example would help.