Relevant System: Gentoo, opencv-3.3.0-r4, libkface-17.08.3

The package fails to compile with the following error:
[ 54%] Building CXX object src/CMakeFiles/KF5KFace.dir/database/core/databasecorebackend.cpp.o
cd /var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3_build/src && /usr/bin/x86_64-pc-linux-gnu-g++ -DKF5KFace_EXPORTS -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NO_CAST_FROM_ASCII -DQT_NO_CAST_FROM_BYTEARRAY -DQT_NO_CAST_TO_ASCII -DQT_NO_DEBUG -DQT_NO_SIGNALS_SLOTS_KEYWORDS -DQT_NO_URL_CAST_FROM_STRING -DQT_SQL_LIB -DQT_USE_FAST_OPERATOR_PLUS -DQT_USE_QSTRINGBUILDER -DQT_WIDGETS_LIB -DQT_XML_LIB -D_GNU_SOURCE -D_LARGEFILE64_SOURCE -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3_build/src -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3_build/src/KF5KFace_autogen/include -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3_build/src/.. -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/database -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/database/core -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/detection -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph -I/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/alignment-congealing -isystem /usr/include/opencv -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtGui -isystem /usr/include/qt5/QtCore -isystem /usr/lib64/qt5/mkspecs/linux-g++ -isystem /usr/include/qt5/QtXml -isystem /usr/include/qt5/QtSql -isystem /usr/include/qt5/QtWidgets   -DQT_NO_DEBUG -DNDEBUG -O2 -march=native -fomit-frame-pointer -pipe -std=c++0x -fno-operator-names -Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef -Wnon-virtual-dtor -Woverloaded-virtual -Werror=return-type -Wvla -Wdate-time -pedantic -Wsuggest-override -fexceptions -fPIC -fvisibility=hidden -fvisibility-inlines-hidden   -fPIC -std=gnu++11 -o CMakeFiles/KF5KFace.dir/database/core/databasecorebackend.cpp.o -c /var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/database/core/databasecorebackend.cpp
In file included from /var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph/facerec_borrowed.cpp:41:0:
/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph/facerec_borrowed.h:120:10: warning: ‘virtual void KFaceIface::LBPHFaceRecognizer::train(cv::InputArrayOfArrays, cv::InputArray)’ can be marked override [-Wsuggest-override]
     void train(cv::InputArrayOfArrays src, cv::InputArray labels);
          ^
/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph/facerec_borrowed.h:126:10: warning: ‘virtual void KFaceIface::LBPHFaceRecognizer::update(cv::InputArrayOfArrays, cv::InputArray)’ can be marked override [-Wsuggest-override]
     void update(cv::InputArrayOfArrays src, cv::InputArray labels);
          ^
/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph/facerec_borrowed.h:184:12: warning: ‘virtual double KFaceIface::LBPHFaceRecognizer::getThreshold() const’ can be marked override [-Wsuggest-override]
     double getThreshold() const                          { return m_threshold;            }
            ^
/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph/facerec_borrowed.h:185:10: warning: ‘virtual void KFaceIface::LBPHFaceRecognizer::setThreshold(double)’ can be marked override [-Wsuggest-override]
     void setThreshold(double _threshold)                 { m_threshold = _threshold;      }
          ^
/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph/facerec_borrowed.cpp: In static member function ‘static cv::Ptr<KFaceIface::LBPHFaceRecognizer> KFaceIface::LBPHFaceRecognizer::create(int, int, int, int, double, KFaceIface::LBPHFaceRecognizer::PredictionStatistics)’:
/var/tmp/portage/kde-apps/libkface-17.08.3/work/libkface-17.08.3/src/recognition-opencv-lbph/facerec_borrowed.cpp:553:115: error: invalid new-expression of abstract class type ‘KFaceIface::LBPHFaceRecognizer’
     LBPHFaceRecognizer* const fr = new LBPHFaceRecognizer(radius, neighbors, grid_x, grid_y, threshold, statistics);
                                                                                                                  
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Looks to me like it is related to an incompatibility with opencv.