when I compile caffe file :

.build_debug/lib/libcaffe.so: undefined reference to `cv::imread(cv::String const&, int)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imencode(cv::String const&, cv::_InputArray const&, std::vector<unsigned char, std::allocator<unsigned char> >&, std::vector<int, std::allocator<int> > const&)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imdecode(cv::_InputArray const&, int)'
collect2: error: ld returned 1 exit status
.build_debug/lib/make: libcaffe.so: undefined *** [.build_debug/tools/upgrade_net_proto_text.bin] Error 1
referencemake: *** Waiting for unfinished jobs....
 to `cv.build_debug/lib/libcaffe.so: undefined reference to `cv::imread(cv::String const&, int)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imencode(cv::String const&, cv::_InputArray const&, std::vector<unsigned char, std::allocator<unsigned char> >&, std::vector<int, std::allocator<int> > const&)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imdecode(cv::/_InputArray const&, usrint)/bin/'ld:
warning: libcudart.so.5.5, needed by /usr/local/lib/libopencv_core.so, may conflict withcollect2: error: ld returned 1 exit status
 libcudart.so.7.5
.build_debug/lib/libcaffe.so: undefined reference to `cv::imread(cv::String const&, int)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imencode(cv::String const&, cv::_InputArray const&, std::vector<unsigned char, std::allocator<unsigned char> >&, std::vector.<int, std::allocatorbuild_debug<int> /> constlib&)'/libcaffe.so
.build_debug/: lib/undefined referencelibcaffe.so: to make:  `*** [.build_debug/tools/upgrade_net_proto_binary.bin] Error 1cvundefined: :referenceimread
( tocv:: `String cvconst&::imdecode, int(cv)'
.::_InputArraybuild_debug/ const&lib, int/)'
libcaffe.so: undefined reference to `cv::imencode(cv:collect2: error: ld returned 1 exit status
:String const&, cv::_InputArray const&, std::vector<unsigned char, std::allocator<unsigned char> >&, std::vector<int, std::allocator<int> > const&)'
.build_debug/lib/libcaffe.so: make: undefined reference*** [.build_debug/tools/caffe.bin] Error 1 to
 `cv::imdecode(cv::_InputArray const&, int)'
::imread(cv::String const&, int)'
collect2: error: ld returned 1 exit status
.build_debug/lib/libcaffe.so: undefined reference to `cv::imencode(cv::String const&, cv::_InputArray const&, std::vector<unsigned char, std::allocator<unsigned char> >&, std::vector<int, std::allocator<int> > const&)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imdecode(cv::_InputArray const&, int)'
make: *** [.build_debug/tools/extract_features.bin] Error 1collect2: error: ld returned 1 exit status

make: *** [.build_debug/tools/compute_image_mean.bin] Error 1
.build_debug/lib/libcaffe.so: undefined reference to `cv::imread(cv::String const&, int)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imencode(cv::String const&, cv::_InputArray const&, std::vector<unsigned char, std::allocator<unsigned char> >&, std::vector<int, std::allocator<int> > const&)'
.build_debug/lib/libcaffe.so: undefined reference to `cv::imdecode(cv::_InputArray const&, int)'
collect2: error: ld returned 1 exit status
make: *** [.build_debug/tools/convert_imageset.bin] Error 1
wangxiao@wangxiao-GTX980:/media/wangxiao/247317a3-e6b5-45d4-81d1-956930526746/---------------/caffe-master$

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
Answer: https://github.com/BVLC/caffe/issues/2348
But if you delete all the file in build(rm -rf ./build/*) before "make all",you will success.I just success
so, just input the code and make all -j8 again.

well, same problem... and change the following code in Makefile:
>>>>>>>>  LIBRARIES += opencv_core opencv_highgui opencv_imgproc
into :    LIBRARIES += opencv_core opencv_highgui opencv_imgproc opencv_imgcodecs

yeah, just add the: opencv_imgcodecs  at last.  and it really worked.

thus, the following steps may needed:

1. change the Makefile :

　　add "opencv_imagecodecs" to the last of LIBRARIES += opencv_core opencv_highgui opencv_imgproc;

2. remove the files in build :

　　rm -rf ./build/*

3. make all -j8