CI would probably help to catch these problems early.
/usr/bin/ld: CMakeFiles/Demo_FEX.dir/Demo_FEX.cpp.o: in function `main':
Demo FEX.cpp:(.text+0x118): undefined reference to `DirectGraphicalModels::fex::CCoordinate::get(cv::Mat const&, DirectGraphicalModels::fex::coordinateType)'
/usr/bin/ld: Demo FEX.cpp:(.text+0x229): undefined reference to `DirectGraphicalModels::fex::CCommonFeatureExtractor::invert() const'
/usr/bin/ld: CMakeFiles/Demo_FEX.dir/Demo_FEX.cpp.o: in function `DirectGraphicalModels::fex::CCommonFeatureExtractor::getIntensity(cv::Scalar_<double>) const':
Demo FEX.cpp:(.text._ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor12getIntensityEN2cv7Scalar_IdEE[_ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor12getIntensityEN2cv7Scalar_IdEE]+0x6a): undefined reference to `DirectGraphicalModels::fex::CIntensity::get(cv::Mat const&, cv::Scalar_<double>)'
/usr/bin/ld: CMakeFiles/Demo_FEX.dir/Demo_FEX.cpp.o: in function `DirectGraphicalModels::fex::CCommonFeatureExtractor::getSaturation() const':
Demo FEX.cpp:(.text._ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor13getSaturationEv[_ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor13getSaturationEv]+0x46): undefined reference to `DirectGraphicalModels::fex::CHSV::get(cv::Mat const&)'
/usr/bin/ld: Demo FEX.cpp:(.text._ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor13getSaturationEv[_ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor13getSaturationEv]+0x77): undefined reference to `DirectGraphicalModels::fex::CCommonFeatureExtractor::getChannel(int) const'
/usr/bin/ld: CMakeFiles/Demo_FEX.dir/Demo_FEX.cpp.o: in function `DirectGraphicalModels::fex::CCommonFeatureExtractor::getNDVI(unsigned char) const':
Demo FEX.cpp:(.text._ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor7getNDVIEh[_ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor7getNDVIEh]+0x52): undefined reference to `DirectGraphicalModels::fex::CNDVI::get(cv::Mat const&, unsigned char)'
/usr/bin/ld: CMakeFiles/Demo_FEX.dir/Demo_FEX.cpp.o: in function `DirectGraphicalModels::fex::CCommonFeatureExtractor::getVariance(DirectGraphicalModels::fex::SqNeighbourhood) const':
Demo FEX.cpp:(.text._ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor11getVarianceENS0_15SqNeighbourhoodE[_ZNK21DirectGraphicalModels3fex23CCommonFeatureExtractor11getVarianceENS0_15SqNeighbourhoodE]+0x5f): undefined reference to `DirectGraphicalModels::fex::CVariance::get(cv::Mat const&, DirectGraphicalModels::fex::SqNeighbourhood)'
collect2: error: ld returned 1 exit status
make[2]: *** [demos/CMakeFiles/Demo_FEX.dir/build.make:92: bin/Demo FEX] Error 1
make[1]: *** [CMakeFiles/Makefile2:413: demos/CMakeFiles/Demo_FEX.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....