A recent poppler upgrade broke both this ebuild and net-print/cups-filters-1.16.0 (see #626800). Someone suggested including -std=c++11 as a build option for cups-filters if you're still on gcc 5.x; that worked. After rebuilding cups-filters, gdal was up next. It also wouldn't rebuild at first, but including -std=c++11 fixed it. I suspect that anything that builds against poppler will need this fix if building on gcc 5.x.
*** This bug has been marked as a duplicate of bug 626844 ***