I've tested this with cups-filters 1.17.9, 1.19.0, and 1.20.0. Each of them fails the configure stage when building against qpdf >= 7; the error is "missing libqpdf." The easiest answer I've found is to add the following to the DEPEND line:
I thought there was another bug for this somewhere but I cannot find it. I believe what's going on is that this new qpdf-7 uses pkgconfig to find its libjpeg dependencies, but only media-libs/libjpeg-turbo or >=media-libs/jpeg-9 provide that pkgconfig file. If you have media-libs/jpeg-8 or earlier, it'll cause a configure failure here.
Are you using stable media-libs/jpeg-8d-r1 as your virtual/jpeg provider?
media-libs/jpeg-8d-r1 is satisfying virtual/jpeg.
*** This bug has been marked as a duplicate of bug 646366 ***