Log Message:
(print/pdf2djvu) Fix build, poppler-20.12 removed poppler-splash

https://openbsdmailbox.blogspot.com/2020/12/graphicspdf2djvu-unbreak-for-poppler.html
the pdf2djvu configure script performs checks against poppler-splash,
but poppler-splash.pc had been removed in poppler-20.12. Check against
poppler itself to unbreak the build.