This is a followup to commit 95bbaa02aa.
See <https://bugs.gnu.org/27621> for more information.
Poppler 0.56.0's ABI is not compatible with Poppler 0.52.0, so it's not
possible to graft the newer version in place of the older one.
This change leaves CVE-2017-9775 unfixed for now.
* gnu/packages/patches/poppler-CVE-2017-9776.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/pdf.scm (poppler-0.56.0): Replace with ...
(poppler/fixed): ... new variable.
(poppler)[replacement]: Replaced with poppler/fixed.
Leo Famulari